[CE] Support multiple assignees for merge requestsosw-multi-assignees-merge-requests

Backports https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/10161
(code out of ee/ folder).

58 files changed, 810 insertions, 408 deletions

diff --git a/spec/controllers/projects/merge_requests_controller_spec.rb b/spec/controllers/projects/merge_requests_controller_spec.rb
index 017162519d8..a125e470522 100644
--- a/spec/controllers/projects/merge_requests_controller_spec.rb
+++ b/spec/controllers/projects/merge_requests_controller_spec.rb
@@ -238,11 +238,11 @@ describe Projects::MergeRequestsController do
         assignee = create(:user)
         project.add_developer(assignee)
 
-        update_merge_request({ assignee_id: assignee.id }, format: :json)
+        update_merge_request({ assignee_ids: [assignee.id] }, format: :json)
+
         body = JSON.parse(response.body)
 
-        expect(body['assignee'].keys)
-          .to match_array(%w(name username avatar_url id state web_url))
+        expect(body['assignees']).to all(include(*%w(name username avatar_url id state web_url)))
       end
     end
 
diff --git a/spec/features/dashboard/issuables_counter_spec.rb b/spec/features/dashboard/issuables_counter_spec.rb
index fbc2e5cc3d3..50b71368e13 100644
--- a/spec/features/dashboard/issuables_counter_spec.rb
+++ b/spec/features/dashboard/issuables_counter_spec.rb
@@ -8,7 +8,7 @@ describe 'Navigation bar counter', :use_clean_rails_memory_store_caching do
 
   before do
     issue.assignees = [user]
-    merge_request.update(assignee: user)
+    merge_request.update(assignees: [user])
     sign_in(user)
   end
 
@@ -33,7 +33,7 @@ describe 'Navigation bar counter', :use_clean_rails_memory_store_caching do
 
     expect_counters('merge_requests', '1')
 
-    merge_request.update(assignee: nil)
+    merge_request.update(assignees: [])
 
     user.invalidate_cache_counts
 
diff --git a/spec/features/dashboard/merge_requests_spec.rb b/spec/features/dashboard/merge_requests_spec.rb
index 4965770605a..0c6713f623c 100644
--- a/spec/features/dashboard/merge_requests_spec.rb
+++ b/spec/features/dashboard/merge_requests_spec.rb
@@ -48,14 +48,14 @@ describe 'Dashboard Merge Requests' do
 
     let!(:assigned_merge_request) do
       create(:merge_request,
-        assignee: current_user,
+        assignees: [current_user],
         source_project: project,
         author: create(:user))
     end
 
     let!(:assigned_merge_request_from_fork) do
       create(:merge_request,
-              source_branch: 'markdown', assignee: current_user,
+              source_branch: 'markdown', assignees: [current_user],
               target_project: public_project, source_project: forked_project,
               author: create(:user))
     end
diff --git a/spec/features/groups/merge_requests_spec.rb b/spec/features/groups/merge_requests_spec.rb
index 54a8016c157..e1bc4eca619 100644
--- a/spec/features/groups/merge_requests_spec.rb
+++ b/spec/features/groups/merge_requests_spec.rb
@@ -38,7 +38,7 @@ describe 'Group merge requests page' do
 
   context 'when merge request assignee to user' do
     before do
-      issuable.update!(assignee: user)
+      issuable.update!(assignees: [user])
 
       visit path
     end
diff --git a/spec/features/issues/form_spec.rb b/spec/features/issues/form_spec.rb
index 26c781350e5..6fa2ad8711f 100644
--- a/spec/features/issues/form_spec.rb
+++ b/spec/features/issues/form_spec.rb
@@ -30,8 +30,8 @@ describe 'New/edit issue', :js do
         # the original method, resulting in infinite recursion when called.
         # This is likely a bug with helper modules included into dynamically generated view classes.
         # To work around this, we have to hold on to and call to the original implementation manually.
-        original_issue_dropdown_options = FormHelper.instance_method(:issue_assignees_dropdown_options)
-        allow_any_instance_of(FormHelper).to receive(:issue_assignees_dropdown_options).and_wrap_original do |original, *args|
+        original_issue_dropdown_options = FormHelper.instance_method(:assignees_dropdown_options)
+        allow_any_instance_of(FormHelper).to receive(:assignees_dropdown_options).and_wrap_original do |original, *args|
           options = original_issue_dropdown_options.bind(original.receiver).call(*args)
           options[:data][:per_page] = 2
 
diff --git a/spec/features/merge_request/user_creates_merge_request_spec.rb b/spec/features/merge_request/user_creates_merge_request_spec.rb
index ea2bb1503bb..bcc11217389 100644
--- a/spec/features/merge_request/user_creates_merge_request_spec.rb
+++ b/spec/features/merge_request/user_creates_merge_request_spec.rb
@@ -68,15 +68,15 @@ describe "User creates a merge request", :js do
         fill_in("Title", with: title)
       end
 
-      click_button("Assignee")
-
       expect(find(".js-assignee-search")["data-project-id"]).to eq(project.id.to_s)
+      find('.js-assignee-search').click
 
       page.within(".dropdown-menu-user") do
         expect(page).to have_content("Unassigned")
                    .and have_content(user.name)
                    .and have_content(project.users.first.name)
       end
+      find('.js-assignee-search').click
 
       click_button("Submit merge request")
 
diff --git a/spec/features/merge_request/user_creates_mr_spec.rb b/spec/features/merge_request/user_creates_mr_spec.rb
index c169a68cd1c..c9dedab048a 100644
--- a/spec/features/merge_request/user_creates_mr_spec.rb
+++ b/spec/features/merge_request/user_creates_mr_spec.rb
@@ -1,11 +1,18 @@
 require 'rails_helper'
 
 describe 'Merge request > User creates MR' do
-  it_behaves_like 'a creatable merge request'
+  include ProjectForksHelper
 
-  context 'from a forked project' do
-    include ProjectForksHelper
+  before do
+    stub_licensed_features(multiple_merge_request_assignees: false)
+  end
 
+  context 'non-fork merge request' do
+    include_context 'merge request create context'
+    it_behaves_like 'a creatable merge request'
+  end
+
+  context 'from a forked project' do
     let(:canonical_project) { create(:project, :public, :repository) }
 
     let(:source_project) do
@@ -15,6 +22,7 @@ describe 'Merge request > User creates MR' do
     end
 
     context 'to canonical project' do
+      include_context 'merge request create context'
       it_behaves_like 'a creatable merge request'
     end
 
@@ -25,6 +33,7 @@ describe 'Merge request > User creates MR' do
           namespace: user.namespace)
       end
 
+      include_context 'merge request create context'
       it_behaves_like 'a creatable merge request'
     end
   end
diff --git a/spec/features/merge_request/user_edits_mr_spec.rb b/spec/features/merge_request/user_edits_mr_spec.rb
index 3152707136c..25979513ead 100644
--- a/spec/features/merge_request/user_edits_mr_spec.rb
+++ b/spec/features/merge_request/user_edits_mr_spec.rb
@@ -1,13 +1,21 @@
-require 'rails_helper'
+require 'spec_helper'
 
 describe 'Merge request > User edits MR' do
   include ProjectForksHelper
 
-  it_behaves_like 'an editable merge request'
+  before do
+    stub_licensed_features(multiple_merge_request_assignees: false)
+  end
+
+  context 'non-fork merge request' do
+    include_context 'merge request edit context'
+    it_behaves_like 'an editable merge request'
+  end
 
   context 'for a forked project' do
-    it_behaves_like 'an editable merge request' do
-      let(:source_project) { fork_project(target_project, nil, repository: true) }
-    end
+    let(:source_project) { fork_project(target_project, nil, repository: true) }
+
+    include_context 'merge request edit context'
+    it_behaves_like 'an editable merge request'
   end
 end
diff --git a/spec/features/merge_requests/user_filters_by_assignees_spec.rb b/spec/features/merge_requests/user_filters_by_assignees_spec.rb
index d6c770c93f1..0cbf1bcae30 100644
--- a/spec/features/merge_requests/user_filters_by_assignees_spec.rb
+++ b/spec/features/merge_requests/user_filters_by_assignees_spec.rb
@@ -7,7 +7,7 @@ describe 'Merge Requests > User filters by assignees', :js do
   let(:user)    { project.creator }
 
   before do
-    create(:merge_request, assignee: user, title: 'Bugfix1', source_project: project, target_project: project, source_branch: 'bugfix1')
+    create(:merge_request, assignees: [user], title: 'Bugfix1', source_project: project, target_project: project, source_branch: 'bugfix1')
     create(:merge_request, title: 'Bugfix2', source_project: project, target_project: project, source_branch: 'bugfix2')
 
     sign_in(user)
diff --git a/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb b/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
index 1615899a047..4627931f26a 100644
--- a/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
+++ b/spec/features/merge_requests/user_filters_by_multiple_criteria_spec.rb
@@ -10,7 +10,7 @@ describe 'Merge requests > User filters by multiple criteria', :js do
 
   before do
     sign_in(user)
-    mr = create(:merge_request, title: 'Bugfix2', author: user, assignee: user, source_project: project, target_project: project, milestone: milestone)
+    mr = create(:merge_request, title: 'Bugfix2', author: user, assignees: [user], source_project: project, target_project: project, milestone: milestone)
     mr.labels << wontfix
 
     visit project_merge_requests_path(project)
diff --git a/spec/features/merge_requests/user_lists_merge_requests_spec.rb b/spec/features/merge_requests/user_lists_merge_requests_spec.rb
index c691011b9ca..bd91fae1453 100644
--- a/spec/features/merge_requests/user_lists_merge_requests_spec.rb
+++ b/spec/features/merge_requests/user_lists_merge_requests_spec.rb
@@ -12,7 +12,7 @@ describe 'Merge requests > User lists merge requests' do
                   title: 'fix',
                   source_project: project,
                   source_branch: 'fix',
-                  assignee: user,
+                  assignees: [user],
                   milestone: create(:milestone, project: project, due_date: '2013-12-11'),
                   created_at: 1.minute.ago,
                   updated_at: 1.minute.ago)
@@ -20,7 +20,7 @@ describe 'Merge requests > User lists merge requests' do
            title: 'markdown',
            source_project: project,
            source_branch: 'markdown',
-           assignee: user,
+           assignees: [user],
            milestone: create(:milestone, project: project, due_date: '2013-12-12'),
            created_at: 2.minutes.ago,
            updated_at: 2.minutes.ago)
diff --git a/spec/features/merge_requests/user_mass_updates_spec.rb b/spec/features/merge_requests/user_mass_updates_spec.rb
index e535c7e5811..c2dd105324d 100644
--- a/spec/features/merge_requests/user_mass_updates_spec.rb
+++ b/spec/features/merge_requests/user_mass_updates_spec.rb
@@ -54,8 +54,7 @@ describe 'Merge requests > User mass updates', :js do
 
     describe 'remove assignee' do
       before do
-        merge_request.assignee = user
-        merge_request.save
+        merge_request.assignees = [user]
         visit project_merge_requests_path(project)
       end
 
diff --git a/spec/features/search/user_uses_header_search_field_spec.rb b/spec/features/search/user_uses_header_search_field_spec.rb
index 444de26733f..1cc47cd6bd1 100644
--- a/spec/features/search/user_uses_header_search_field_spec.rb
+++ b/spec/features/search/user_uses_header_search_field_spec.rb
@@ -36,7 +36,7 @@ describe 'User uses header search field' do
     end
 
     context 'when clicking merge requests' do
-      let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignee: user) }
+      let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignees: [user]) }
 
       it 'shows assigned merge requests' do
         find('.search-input-container .dropdown-menu').click_link('Merge requests assigned to me')
@@ -100,7 +100,7 @@ describe 'User uses header search field' do
       end
 
       context 'when clicking merge requests' do
-        let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignee: user) }
+        let!(:merge_request) { create(:merge_request, source_project: project, author: user, assignees: [user]) }
 
         it 'shows assigned merge requests' do
           find('.dropdown-menu').click_link('Merge requests assigned to me')
diff --git a/spec/finders/issues_finder_spec.rb b/spec/finders/issues_finder_spec.rb
index fe53fabe54c..6e6dc334354 100644
--- a/spec/finders/issues_finder_spec.rb
+++ b/spec/finders/issues_finder_spec.rb
@@ -13,60 +13,32 @@ describe IssuesFinder do
         expect(issues).to contain_exactly(issue1, issue2, issue3, issue4)
       end
 
-      context 'filtering by assignee ID' do
-        let(:params) { { assignee_id: user.id } }
+      context 'assignee filtering' do
+        let(:issuables) { issues }
 
-        it 'returns issues assigned to that user' do
-          expect(issues).to contain_exactly(issue1, issue2)
-        end
-      end
-
-      context 'filtering by assignee usernames' do
-        set(:user3) { create(:user) }
-        let(:params) { { assignee_username: [user2.username, user3.username] } }
-
-        before do
-          project2.add_developer(user3)
-
-          issue3.assignees = [user2, user3]
-        end
-
-        it 'returns issues assigned to those users' do
-          expect(issues).to contain_exactly(issue3)
-        end
-      end
-
-      context 'filtering by no assignee' do
-        let(:params) { { assignee_id: 'None' } }
-
-        it 'returns issues not assigned to any assignee' do
-          expect(issues).to contain_exactly(issue4)
-        end
-
-        it 'returns issues not assigned to any assignee' do
-          params[:assignee_id] = 0
-
-          expect(issues).to contain_exactly(issue4)
+        it_behaves_like 'assignee ID filter' do
+          let(:params) { { assignee_id: user.id } }
+          let(:expected_issuables) { [issue1, issue2] }
         end
 
-        it 'returns issues not assigned to any assignee' do
-          params[:assignee_id] = 'none'
+        it_behaves_like 'assignee username filter' do
+          before do
+            project2.add_developer(user3)
+            issue3.assignees = [user2, user3]
+          end
 
-          expect(issues).to contain_exactly(issue4)
+          set(:user3) { create(:user) }
+          let(:params) { { assignee_username: [user2.username, user3.username] } }
+          let(:expected_issuables) { [issue3] }
         end
-      end
 
-      context 'filtering by any assignee' do
-        let(:params) { { assignee_id: 'Any' } }
-
-        it 'returns issues assigned to any assignee' do
-          expect(issues).to contain_exactly(issue1, issue2, issue3)
+        it_behaves_like 'no assignee filter' do
+          set(:user3) { create(:user) }
+          let(:expected_issuables) { [issue4] }
         end
 
-        it 'returns issues assigned to any assignee' do
-          params[:assignee_id] = 'any'
-
-          expect(issues).to contain_exactly(issue1, issue2, issue3)
+        it_behaves_like 'any assignee filter' do
+          let(:expected_issuables) { [issue1, issue2, issue3] }
         end
       end
 
diff --git a/spec/finders/merge_requests_finder_spec.rb b/spec/finders/merge_requests_finder_spec.rb
index f508b9bdb6f..ef002125635 100644
--- a/spec/finders/merge_requests_finder_spec.rb
+++ b/spec/finders/merge_requests_finder_spec.rb
@@ -136,21 +136,50 @@ describe MergeRequestsFinder do
         end
       end
 
-      context 'filtering by group milestone' do
-        let(:group_milestone) { create(:milestone, group: group) }
+      context 'assignee filtering' do
+        let(:issuables) { described_class.new(user, params).execute }
 
-        before do
-          project2.update(namespace: group)
-          merge_request2.update(milestone: group_milestone)
-          merge_request3.update(milestone: group_milestone)
+        it_behaves_like 'assignee ID filter' do
+          let(:params) { { assignee_id: user.id } }
+          let(:expected_issuables) { [merge_request1, merge_request2] }
         end
 
-        it 'returns merge requests assigned to that group milestone' do
-          params = { milestone_title: group_milestone.title }
+        it_behaves_like 'assignee username filter' do
+          before do
+            project2.add_developer(user3)
+            merge_request3.assignees = [user2, user3]
+          end
 
-          merge_requests = described_class.new(user, params).execute
+          set(:user3) { create(:user) }
+          let(:params) { { assignee_username: [user2.username, user3.username] } }
+          let(:expected_issuables) { [merge_request3] }
+        end
+
+        it_behaves_like 'no assignee filter' do
+          set(:user3) { create(:user) }
+          let(:expected_issuables) { [merge_request4, merge_request5] }
+        end
+
+        it_behaves_like 'any assignee filter' do
+          let(:expected_issuables) { [merge_request1, merge_request2, merge_request3] }
+        end
+
+        context 'filtering by group milestone' do
+          let(:group_milestone) { create(:milestone, group: group) }
+
+          before do
+            project2.update(namespace: group)
+            merge_request2.update(milestone: group_milestone)
+            merge_request3.update(milestone: group_milestone)
+          end
+
+          it 'returns merge requests assigned to that group milestone' do
+            params = { milestone_title: group_milestone.title }
 
-          expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+            merge_requests = described_class.new(user, params).execute
+
+            expect(merge_requests).to contain_exactly(merge_request2, merge_request3)
+          end
         end
       end
 
diff --git a/spec/fixtures/api/schemas/entities/merge_request_basic.json b/spec/fixtures/api/schemas/entities/merge_request_basic.json
index 3006b482d41..88a600398b1 100644
--- a/spec/fixtures/api/schemas/entities/merge_request_basic.json
+++ b/spec/fixtures/api/schemas/entities/merge_request_basic.json
@@ -6,14 +6,14 @@
     "source_branch_exists": { "type": "boolean" },
     "merge_error": { "type": ["string", "null"] },
     "rebase_in_progress": { "type": "boolean" },
-    "assignee_id": { "type": ["integer", "null"] },
     "allow_collaboration": { "type": "boolean"},
     "allow_maintainer_to_push": { "type": "boolean"},
-    "assignee": {
-      "oneOf": [
-        { "type": "null" },
-        { "$ref": "user.json" }
-      ]
+    "assignees": {
+      "type": ["array"],
+      "items": {
+        "type": "object",
+        "$ref": "../public_api/v4/user/basic.json"
+      }
     },
     "milestone": {
       "type": [ "object", "null" ]
diff --git a/spec/fixtures/api/schemas/public_api/v4/merge_request.json b/spec/fixtures/api/schemas/public_api/v4/merge_request.json
index 918f2c4b47d..a423bf70b69 100644
--- a/spec/fixtures/api/schemas/public_api/v4/merge_request.json
+++ b/spec/fixtures/api/schemas/public_api/v4/merge_request.json
@@ -64,6 +64,11 @@
         },
         "additionalProperties": false
       },
+      "assignees": {
+        "items": {
+          "$ref": "./merge_request.json"
+        }
+      },
       "source_project_id": { "type": "integer" },
       "target_project_id": { "type": "integer" },
       "labels": {
diff --git a/spec/javascripts/sidebar/assignees_spec.js b/spec/javascripts/sidebar/assignees_spec.js
index 57b16b12cb0..47fee5d2b21 100644
--- a/spec/javascripts/sidebar/assignees_spec.js
+++ b/spec/javascripts/sidebar/assignees_spec.js
@@ -132,9 +132,94 @@ describe('Assignee component', () => {
         -1,
       );
     });
+
+    it('has correct "cannot merge" tooltip when user cannot merge', () => {
+      const user = Object.assign({}, UsersMock.user, { can_merge: false });
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users: [user],
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('Cannot merge');
+    });
   });
 
   describe('Two or more assignees/users', () => {
+    it('has correct "cannot merge" tooltip when one user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(3);
+      users[0].can_merge = true;
+      users[1].can_merge = false;
+      users[2].can_merge = false;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('1/3 can merge');
+    });
+
+    it('has correct "cannot merge" tooltip when no user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(2);
+      users[0].can_merge = false;
+      users[1].can_merge = false;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('No one can merge');
+    });
+
+    it('has correct "cannot merge" tooltip when more than one user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(3);
+      users[0].can_merge = false;
+      users[1].can_merge = true;
+      users[2].can_merge = true;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual('2/3 can merge');
+    });
+
+    it('has no "cannot merge" tooltip when every user can merge', () => {
+      const users = UsersMockHelper.createNumberRandomUsers(2);
+      users[0].can_merge = true;
+      users[1].can_merge = true;
+
+      component = new AssigneeComponent({
+        propsData: {
+          rootPath: 'http://localhost:3000/',
+          users,
+          editable: true,
+          issuableType: 'merge_request',
+        },
+      }).$mount();
+
+      expect(component.mergeNotAllowedTooltipMessage).toEqual(null);
+    });
+
     it('displays two assignee icons when collapsed', () => {
       const users = UsersMockHelper.createNumberRandomUsers(2);
       component = new AssigneeComponent({
diff --git a/spec/lib/gitlab/hook_data/issuable_builder_spec.rb b/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
index 26529c4759d..569d5dcc757 100644
--- a/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
+++ b/spec/lib/gitlab/hook_data/issuable_builder_spec.rb
@@ -97,13 +97,13 @@ describe Gitlab::HookData::IssuableBuilder do
     end
 
     context 'merge_request is assigned' do
-      let(:merge_request) { create(:merge_request, assignee: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user]) }
       let(:data) { described_class.new(merge_request).build(user: user) }
 
       it 'returns correct hook data' do
         expect(data[:object_attributes]['assignee_id']).to eq(user.id)
-        expect(data[:assignee]).to eq(user.hook_attrs)
-        expect(data).not_to have_key(:assignees)
+        expect(data[:assignees].first).to eq(user.hook_attrs)
+        expect(data).not_to have_key(:assignee)
       end
     end
   end
diff --git a/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb b/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
index 9ce697adbba..39f80f92fa6 100644
--- a/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
+++ b/spec/lib/gitlab/hook_data/merge_request_builder_spec.rb
@@ -10,6 +10,7 @@ describe Gitlab::HookData::MergeRequestBuilder do
     it 'includes safe attribute' do
       %w[
         assignee_id
+        assignee_ids
         author_id
         created_at
         description
diff --git a/spec/lib/gitlab/import_export/all_models.yml b/spec/lib/gitlab/import_export/all_models.yml
index e418516569a..ed557ffd4e3 100644
--- a/spec/lib/gitlab/import_export/all_models.yml
+++ b/spec/lib/gitlab/import_export/all_models.yml
@@ -102,6 +102,7 @@ merge_requests:
 - merge_request_pipelines
 - merge_request_assignees
 - suggestions
+- assignees
 merge_request_diff:
 - merge_request
 - merge_request_diff_commits
@@ -336,6 +337,9 @@ push_event_payload:
 issue_assignees:
 - issue
 - assignee
+merge_request_assignees:
+- merge_request
+- assignee
 lfs_file_locks:
 - user
 project_badges:
diff --git a/spec/lib/gitlab/import_export/safe_model_attributes.yml b/spec/lib/gitlab/import_export/safe_model_attributes.yml
index d0ed588f05f..06995604a24 100644
--- a/spec/lib/gitlab/import_export/safe_model_attributes.yml
+++ b/spec/lib/gitlab/import_export/safe_model_attributes.yml
@@ -621,3 +621,7 @@ Suggestion:
 - outdated
 - lines_above
 - lines_below
+MergeRequestAssignee:
+- id
+- user_id
+- merge_request_id
diff --git a/spec/lib/gitlab/issuable_metadata_spec.rb b/spec/lib/gitlab/issuable_metadata_spec.rb
index 6ec86163233..916f3876a8e 100644
--- a/spec/lib/gitlab/issuable_metadata_spec.rb
+++ b/spec/lib/gitlab/issuable_metadata_spec.rb
@@ -19,7 +19,7 @@ describe Gitlab::IssuableMetadata do
     let!(:closed_issue) { create(:issue, state: :closed, author: user, project: project) }
     let!(:downvote) { create(:award_emoji, :downvote, awardable: closed_issue) }
     let!(:upvote) { create(:award_emoji, :upvote, awardable: issue) }
-    let!(:merge_request) { create(:merge_request, :simple, author: user, assignee: user, source_project: project, target_project: project, title: "Test") }
+    let!(:merge_request) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project, target_project: project, title: "Test") }
     let!(:closing_issues) { create(:merge_requests_closing_issues, issue: issue, merge_request: merge_request) }
 
     it 'aggregates stats on issues' do
@@ -39,7 +39,7 @@ describe Gitlab::IssuableMetadata do
   end
 
   context 'merge requests' do
-    let!(:merge_request) { create(:merge_request, :simple, author: user, assignee: user, source_project: project, target_project: project, title: "Test") }
+    let!(:merge_request) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project, target_project: project, title: "Test") }
     let!(:merge_request_closed) { create(:merge_request, state: "closed", source_project: project, target_project: project, title: "Closed Test") }
     let!(:downvote) { create(:award_emoji, :downvote, awardable: merge_request) }
     let!(:upvote) { create(:award_emoji, :upvote, awardable: merge_request) }
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 5fa1369c00a..fee1d701e3a 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -19,7 +19,7 @@ describe Notify do
     create(:merge_request, source_project: project,
                            target_project: project,
                            author: current_user,
-                           assignee: assignee,
+                           assignees: [assignee],
                            description: 'Awesome description')
   end
 
@@ -275,7 +275,7 @@ describe Notify do
 
     context 'for merge requests' do
       describe 'that are new' do
-        subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id) }
+        subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id) }
 
         it_behaves_like 'an assignee email'
         it_behaves_like 'an email starting a new thread with reply-by-email enabled' do
@@ -300,7 +300,7 @@ describe Notify do
         end
 
         context 'when sent with a reason' do
-          subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id, NotificationReason::ASSIGNED) }
+          subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id, NotificationReason::ASSIGNED) }
 
           it_behaves_like 'appearance header and footer enabled'
           it_behaves_like 'appearance header and footer not enabled'
@@ -324,7 +324,7 @@ describe Notify do
 
       describe 'that are reassigned' do
         let(:previous_assignee) { create(:user, name: 'Previous Assignee') }
-        subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id) }
+        subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id) }
 
         it_behaves_like 'a multiple recipients email'
         it_behaves_like 'an answer to an existing thread with reply-by-email enabled' do
@@ -351,7 +351,7 @@ describe Notify do
         end
 
         context 'when sent with a reason' do
-          subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, NotificationReason::ASSIGNED) }
+          subject { described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, NotificationReason::ASSIGNED) }
 
           it_behaves_like 'appearance header and footer enabled'
           it_behaves_like 'appearance header and footer not enabled'
@@ -364,11 +364,11 @@ describe Notify do
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(NotificationReason::ASSIGNED)
             is_expected.to have_body_text(text)
 
-            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, NotificationReason::MENTIONED)
+            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, NotificationReason::MENTIONED)
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(NotificationReason::MENTIONED)
             expect(new_subject).to have_body_text(text)
 
-            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, previous_assignee.id, current_user.id, nil)
+            new_subject = described_class.reassigned_merge_request_email(recipient.id, merge_request.id, [previous_assignee.id], current_user.id, nil)
             text = EmailsHelper.instance_method(:notification_reason_text).bind(self).call(nil)
             expect(new_subject).to have_body_text(text)
           end
@@ -376,7 +376,7 @@ describe Notify do
       end
 
       describe 'that are new with a description' do
-        subject { described_class.new_merge_request_email(merge_request.assignee_id, merge_request.id) }
+        subject { described_class.new_merge_request_email(merge_request.assignee_ids.first, merge_request.id) }
 
         it_behaves_like 'it should show Gmail Actions View Merge request link'
         it_behaves_like "an unsubscribeable thread"
@@ -476,7 +476,7 @@ describe Notify do
                  source_project: project,
                  target_project: project,
                  author: current_user,
-                 assignee: assignee,
+                 assignees: [assignee],
                  description: 'Awesome description')
         end
 
diff --git a/spec/models/ci/pipeline_spec.rb b/spec/models/ci/pipeline_spec.rb
index 83b0f172f03..f3e78630c1b 100644
--- a/spec/models/ci/pipeline_spec.rb
+++ b/spec/models/ci/pipeline_spec.rb
@@ -684,12 +684,12 @@ describe Ci::Pipeline, :mailer do
                source_branch: 'feature',
                target_project: project,
                target_branch: 'master',
-               assignee: assignee,
+               assignees: assignees,
                milestone: milestone,
                labels: labels)
       end
 
-      let(:assignee) { create(:user) }
+      let(:assignees) { create_list(:user, 2) }
       let(:milestone) { create(:milestone, project: project) }
       let(:labels) { create_list(:label, 2) }
 
@@ -710,7 +710,7 @@ describe Ci::Pipeline, :mailer do
             'CI_MERGE_REQUEST_SOURCE_BRANCH_NAME' => merge_request.source_branch.to_s,
             'CI_MERGE_REQUEST_SOURCE_BRANCH_SHA' => pipeline.source_sha.to_s,
             'CI_MERGE_REQUEST_TITLE' => merge_request.title,
-            'CI_MERGE_REQUEST_ASSIGNEES' => assignee.username,
+            'CI_MERGE_REQUEST_ASSIGNEES' => merge_request.assignee_username_list,
             'CI_MERGE_REQUEST_MILESTONE' => milestone.title,
             'CI_MERGE_REQUEST_LABELS' => labels.map(&:title).join(','))
       end
@@ -730,7 +730,7 @@ describe Ci::Pipeline, :mailer do
       end
 
       context 'without assignee' do
-        let(:assignee) { nil }
+        let(:assignees) { [] }
 
         it 'does not expose assignee variable' do
           expect(subject.to_hash.keys).not_to include('CI_MERGE_REQUEST_ASSIGNEES')
diff --git a/spec/models/concerns/deprecated_assignee_spec.rb b/spec/models/concerns/deprecated_assignee_spec.rb
new file mode 100644
index 00000000000..e394de0aa34
--- /dev/null
+++ b/spec/models/concerns/deprecated_assignee_spec.rb
@@ -0,0 +1,160 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe DeprecatedAssignee do
+  let(:user) { create(:user) }
+
+  describe '#assignee_id=' do
+    it 'creates the merge_request_assignees relation' do
+      merge_request = create(:merge_request, assignee_id: user.id)
+
+      merge_request.reload
+
+      expect(merge_request.merge_request_assignees.count).to eq(1)
+    end
+
+    it 'nullifies the assignee_id column' do
+      merge_request = create(:merge_request, assignee_id: user.id)
+
+      merge_request.reload
+
+      expect(merge_request.read_attribute(:assignee_id)).to be_nil
+    end
+
+    context 'when relation already exists' do
+      it 'overwrites existing assignees' do
+        other_user = create(:user)
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.merge_request_assignees.create!(user_id: user.id)
+        merge_request.merge_request_assignees.create!(user_id: other_user.id)
+
+        expect { merge_request.update!(assignee_id: other_user.id) }
+          .to change { merge_request.reload.merge_request_assignees.count }
+          .from(2).to(1)
+      end
+    end
+  end
+
+  describe '#assignee=' do
+    it 'creates the merge_request_assignees relation' do
+      merge_request = create(:merge_request, assignee: user)
+
+      merge_request.reload
+
+      expect(merge_request.merge_request_assignees.count).to eq(1)
+    end
+
+    it 'nullifies the assignee_id column' do
+      merge_request = create(:merge_request, assignee: user)
+
+      merge_request.reload
+
+      expect(merge_request.read_attribute(:assignee_id)).to be_nil
+    end
+
+    context 'when relation already exists' do
+      it 'overwrites existing assignees' do
+        other_user = create(:user)
+        merge_request = create(:merge_request, assignee: nil)
+        merge_request.merge_request_assignees.create!(user_id: user.id)
+        merge_request.merge_request_assignees.create!(user_id: other_user.id)
+
+        expect { merge_request.update!(assignee: other_user) }
+          .to change { merge_request.reload.merge_request_assignees.count }
+          .from(2).to(1)
+      end
+    end
+  end
+
+  describe '#assignee_id' do
+    it 'returns the first assignee ID' do
+      other_user = create(:user)
+      merge_request = create(:merge_request, assignees: [user, other_user])
+
+      merge_request.reload
+
+      expect(merge_request.assignee_id).to eq(merge_request.assignee_ids.first)
+    end
+  end
+
+  describe '#assignees' do
+    context 'when assignee_id exists and there is no relation' do
+      it 'creates the relation' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignees }.to change { merge_request.merge_request_assignees.count }.from(0).to(1)
+      end
+
+      it 'nullifies the assignee_id' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignees }
+          .to change { merge_request.read_attribute(:assignee_id) }
+          .from(user.id).to(nil)
+      end
+    end
+
+    context 'when DB is read-only' do
+      before do
+        allow(Gitlab::Database).to receive(:read_only?) { true }
+      end
+
+      it 'returns a users relation' do
+        merge_request = create(:merge_request, assignee_id: user.id)
+
+        expect(merge_request.assignees).to be_a(ActiveRecord::Relation)
+        expect(merge_request.assignees).to eq([user])
+      end
+
+      it 'returns an empty relation if no assignee_id is set' do
+        merge_request = create(:merge_request, assignee_id: nil)
+
+        expect(merge_request.assignees).to be_a(ActiveRecord::Relation)
+        expect(merge_request.assignees).to eq([])
+      end
+    end
+  end
+
+  describe '#assignee_ids' do
+    context 'when assignee_id exists and there is no relation' do
+      it 'creates the relation' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignee_ids }.to change { merge_request.merge_request_assignees.count }.from(0).to(1)
+      end
+
+      it 'nullifies the assignee_id' do
+        merge_request = create(:merge_request, assignee_id: nil)
+        merge_request.update_column(:assignee_id, user.id)
+
+        expect { merge_request.assignee_ids }
+          .to change { merge_request.read_attribute(:assignee_id) }
+          .from(user.id).to(nil)
+      end
+    end
+
+    context 'when DB is read-only' do
+      before do
+        allow(Gitlab::Database).to receive(:read_only?) { true }
+      end
+
+      it 'returns a list of user IDs' do
+        merge_request = create(:merge_request, assignee_id: user.id)
+
+        expect(merge_request.assignee_ids).to be_a(Array)
+        expect(merge_request.assignee_ids).to eq([user.id])
+      end
+
+      it 'returns an empty relation if no assignee_id is set' do
+        merge_request = create(:merge_request, assignee_id: nil)
+
+        expect(merge_request.assignee_ids).to be_a(Array)
+        expect(merge_request.assignee_ids).to eq([])
+      end
+    end
+  end
+end
diff --git a/spec/models/concerns/issuable_spec.rb b/spec/models/concerns/issuable_spec.rb
index 27ed298ae08..64f02978d79 100644
--- a/spec/models/concerns/issuable_spec.rb
+++ b/spec/models/concerns/issuable_spec.rb
@@ -502,8 +502,8 @@ describe Issuable do
       let(:user2) { create(:user) }
 
       before do
-        merge_request.update(assignee: user)
-        merge_request.update(assignee: user2)
+        merge_request.update(assignees: [user])
+        merge_request.update(assignees: [user, user2])
         expect(Gitlab::HookData::IssuableBuilder)
           .to receive(:new).with(merge_request).and_return(builder)
       end
@@ -512,8 +512,7 @@ describe Issuable do
         expect(builder).to receive(:build).with(
           user: user,
           changes: hash_including(
-            'assignee_id' => [user.id, user2.id],
-            'assignee' => [user.hook_attrs, user2.hook_attrs]
+            'assignees' => [[user.hook_attrs], [user.hook_attrs, user2.hook_attrs]]
           ))
 
         merge_request.to_hook_data(user, old_associations: { assignees: [user] })
diff --git a/spec/models/event_spec.rb b/spec/models/event_spec.rb
index d192fe70506..e91b5c4c86f 100644
--- a/spec/models/event_spec.rb
+++ b/spec/models/event_spec.rb
@@ -263,7 +263,7 @@ describe Event do
 
     context 'merge request diff note event' do
       let(:project) { create(:project, :public) }
-      let(:merge_request) { create(:merge_request, source_project: project, author: author, assignee: assignee) }
+      let(:merge_request) { create(:merge_request, source_project: project, author: author, assignees: [assignee]) }
       let(:note_on_merge_request) { create(:legacy_diff_note_on_merge_request, noteable: merge_request, project: project) }
       let(:target) { note_on_merge_request }
 
diff --git a/spec/models/merge_request_spec.rb b/spec/models/merge_request_spec.rb
index 6f34ef9c1bc..f61857ea5ff 100644
--- a/spec/models/merge_request_spec.rb
+++ b/spec/models/merge_request_spec.rb
@@ -13,7 +13,7 @@ describe MergeRequest do
     it { is_expected.to belong_to(:target_project).class_name('Project') }
     it { is_expected.to belong_to(:source_project).class_name('Project') }
     it { is_expected.to belong_to(:merge_user).class_name("User") }
-    it { is_expected.to belong_to(:assignee) }
+    it { is_expected.to have_many(:assignees).through(:merge_request_assignees) }
     it { is_expected.to have_many(:merge_request_diffs) }
 
     context 'for forks' do
@@ -181,31 +181,6 @@ describe MergeRequest do
         expect(MergeRequest::Metrics.count).to eq(1)
       end
     end
-
-    describe '#refresh_merge_request_assignees' do
-      set(:user) { create(:user) }
-
-      it 'creates merge request assignees relation upon MR creation' do
-        merge_request = create(:merge_request, assignee: nil)
-
-        expect(merge_request.merge_request_assignees).to be_empty
-
-        expect { merge_request.update!(assignee: user) }
-          .to change { merge_request.reload.merge_request_assignees.count }
-          .from(0).to(1)
-      end
-
-      it 'updates merge request assignees relation upon MR assignee change' do
-        another_user = create(:user)
-        merge_request = create(:merge_request, assignee: user)
-
-        expect { merge_request.update!(assignee: another_user) }
-          .to change { merge_request.reload.merge_request_assignees.first.assignee }
-          .from(user).to(another_user)
-
-        expect(merge_request.merge_request_assignees.count).to eq(1)
-      end
-    end
   end
 
   describe 'respond to' do
@@ -337,34 +312,18 @@ describe MergeRequest do
   describe '#card_attributes' do
     it 'includes the author name' do
       allow(subject).to receive(:author).and_return(double(name: 'Robert'))
-      allow(subject).to receive(:assignee).and_return(nil)
+      allow(subject).to receive(:assignees).and_return([])
 
       expect(subject.card_attributes)
-        .to eq({ 'Author' => 'Robert', 'Assignee' => nil })
+        .to eq({ 'Author' => 'Robert', 'Assignee' => "" })
     end
 
-    it 'includes the assignee name' do
+    it 'includes the assignees name' do
       allow(subject).to receive(:author).and_return(double(name: 'Robert'))
-      allow(subject).to receive(:assignee).and_return(double(name: 'Douwe'))
+      allow(subject).to receive(:assignees).and_return([double(name: 'Douwe'), double(name: 'Robert')])
 
       expect(subject.card_attributes)
-        .to eq({ 'Author' => 'Robert', 'Assignee' => 'Douwe' })
-    end
-  end
-
-  describe '#assignee_ids' do
-    it 'returns an array of the assigned user id' do
-      subject.assignee_id = 123
-
-      expect(subject.assignee_ids).to eq([123])
-    end
-  end
-
-  describe '#assignee_ids=' do
-    it 'sets assignee_id to the last id in the array' do
-      subject.assignee_ids = [123, 456]
-
-      expect(subject.assignee_id).to eq(456)
+        .to eq({ 'Author' => 'Robert', 'Assignee' => 'Douwe and Robert' })
     end
   end
 
@@ -372,7 +331,7 @@ describe MergeRequest do
     let(:user) { create(:user) }
 
     it 'returns true for a user that is assigned to a merge request' do
-      subject.assignee = user
+      subject.assignees = [user]
 
       expect(subject.assignee_or_author?(user)).to eq(true)
     end
@@ -1949,15 +1908,14 @@ describe MergeRequest do
     it 'updates when assignees change' do
       user1 = create(:user)
       user2 = create(:user)
-      mr = create(:merge_request, assignee: user1)
+      mr = create(:merge_request, assignees: [user1])
       mr.project.add_developer(user1)
       mr.project.add_developer(user2)
 
       expect(user1.assigned_open_merge_requests_count).to eq(1)
       expect(user2.assigned_open_merge_requests_count).to eq(0)
 
-      mr.assignee = user2
-      mr.save
+      mr.assignees = [user2]
 
       expect(user1.assigned_open_merge_requests_count).to eq(0)
       expect(user2.assigned_open_merge_requests_count).to eq(1)
diff --git a/spec/models/user_spec.rb b/spec/models/user_spec.rb
index a45a2737b13..d1338e34bb8 100644
--- a/spec/models/user_spec.rb
+++ b/spec/models/user_spec.rb
@@ -2816,9 +2816,9 @@ describe User do
       project = create(:project, :public)
       archived_project = create(:project, :public, :archived)
 
-      create(:merge_request, source_project: project, author: user, assignee: user)
-      create(:merge_request, :closed, source_project: project, author: user, assignee: user)
-      create(:merge_request, source_project: archived_project, author: user, assignee: user)
+      create(:merge_request, source_project: project, author: user, assignees: [user])
+      create(:merge_request, :closed, source_project: project, author: user, assignees: [user])
+      create(:merge_request, source_project: archived_project, author: user, assignees: [user])
 
       expect(user.assigned_open_merge_requests_count(force: true)).to eq 1
     end
diff --git a/spec/requests/api/events_spec.rb b/spec/requests/api/events_spec.rb
index 0ac23505de7..065b16c6221 100644
--- a/spec/requests/api/events_spec.rb
+++ b/spec/requests/api/events_spec.rb
@@ -270,8 +270,8 @@ describe API::Events do
     end
 
     context 'when exists some events' do
-      let(:merge_request1) { create(:merge_request, :closed, author: user, assignee: user, source_project: private_project, title: 'Test') }
-      let(:merge_request2) { create(:merge_request, :closed, author: user, assignee: user, source_project: private_project, title: 'Test') }
+      let(:merge_request1) { create(:merge_request, :closed, author: user, assignees: [user], source_project: private_project, title: 'Test') }
+      let(:merge_request2) { create(:merge_request, :closed, author: user, assignees: [user], source_project: private_project, title: 'Test') }
 
       before do
         create_event(merge_request1)
diff --git a/spec/requests/api/merge_requests_spec.rb b/spec/requests/api/merge_requests_spec.rb
index 7ffa365c651..45818edbf68 100644
--- a/spec/requests/api/merge_requests_spec.rb
+++ b/spec/requests/api/merge_requests_spec.rb
@@ -5,14 +5,15 @@ describe API::MergeRequests do
 
   let(:base_time)   { Time.now }
   set(:user)        { create(:user) }
+  set(:user2)       { create(:user) }
   set(:admin)       { create(:user, :admin) }
   let(:project)     { create(:project, :public, :repository, creator: user, namespace: user.namespace, only_allow_merge_if_pipeline_succeeds: false) }
   let(:milestone)   { create(:milestone, title: '1.0.0', project: project) }
   let(:milestone1)  { create(:milestone, title: '0.9', project: project) }
-  let!(:merge_request) { create(:merge_request, :simple, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time) }
-  let!(:merge_request_closed) { create(:merge_request, state: "closed", milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Closed test", created_at: base_time + 1.second) }
-  let!(:merge_request_merged) { create(:merge_request, state: "merged", author: user, assignee: user, source_project: project, target_project: project, title: "Merged test", created_at: base_time + 2.seconds, merge_commit_sha: '9999999999999999999999999999999999999999') }
-  let!(:merge_request_locked) { create(:merge_request, state: "locked", milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Locked test", created_at: base_time + 1.second) }
+  let!(:merge_request) { create(:merge_request, :simple, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Test", created_at: base_time) }
+  let!(:merge_request_closed) { create(:merge_request, state: "closed", milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Closed test", created_at: base_time + 1.second) }
+  let!(:merge_request_merged) { create(:merge_request, state: "merged", author: user, assignees: [user], source_project: project, target_project: project, title: "Merged test", created_at: base_time + 2.seconds, merge_commit_sha: '9999999999999999999999999999999999999999') }
+  let!(:merge_request_locked) { create(:merge_request, state: "locked", milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Locked test", created_at: base_time + 1.second) }
   let!(:note)       { create(:note_on_merge_request, author: user, project: project, noteable: merge_request, note: "a comment on a MR") }
   let!(:note2)      { create(:note_on_merge_request, author: user, project: project, noteable: merge_request, note: "another comment on a MR") }
   let(:label) { create(:label, title: 'label', color: '#FFAABB', project: project) }
@@ -20,6 +21,9 @@ describe API::MergeRequests do
 
   before do
     project.add_reporter(user)
+    project.add_reporter(user2)
+
+    stub_licensed_features(multiple_merge_request_assignees: false)
   end
 
   shared_context 'with labels' do
@@ -45,9 +49,9 @@ describe API::MergeRequests do
           get api(endpoint_path, user)
         end
 
-        create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: 'Test', created_at: base_time)
+        create(:merge_request, state: 'closed', milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: 'Test', created_at: base_time)
 
-        merge_request = create(:merge_request, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: 'Test', created_at: base_time)
+        merge_request = create(:merge_request, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: 'Test', created_at: base_time)
 
         merge_request.metrics.update!(merged_by: user,
                                       latest_closed_by: user,
@@ -333,7 +337,7 @@ describe API::MergeRequests do
                    state: 'closed',
                    milestone: milestone1,
                    author: user,
-                   assignee: user,
+                   assignees: [user],
                    source_project: project,
                    target_project: project,
                    title: "Test",
@@ -451,7 +455,7 @@ describe API::MergeRequests do
 
     context 'when authenticated' do
       let!(:project2) { create(:project, :public, namespace: user.namespace) }
-      let!(:merge_request2) { create(:merge_request, :simple, author: user, assignee: user, source_project: project2, target_project: project2) }
+      let!(:merge_request2) { create(:merge_request, :simple, author: user, assignees: [user], source_project: project2, target_project: project2) }
       let(:user2) { create(:user) }
 
       it 'returns an array of all merge requests except unauthorized ones' do
@@ -494,7 +498,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by current user if no scope is given' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2)
 
@@ -502,7 +506,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests authored by the given user' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user), params: { author_id: user2.id, scope: :all }
 
@@ -510,7 +514,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to the given user' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user), params: { assignee_id: user2.id, scope: :all }
 
@@ -535,7 +539,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to me' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'assigned_to_me' }
 
@@ -543,7 +547,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests assigned to me (kebab-case)' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user2, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'assigned-to-me' }
 
@@ -551,7 +555,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by me' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'created_by_me' }
 
@@ -559,7 +563,7 @@ describe API::MergeRequests do
       end
 
       it 'returns an array of merge requests created by me (kebab-case)' do
-        merge_request3 = create(:merge_request, :simple, author: user2, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user2, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
 
         get api('/merge_requests', user2), params: { scope: 'created-by-me' }
 
@@ -567,7 +571,7 @@ describe API::MergeRequests do
       end
 
       it 'returns merge requests reacted by the authenticated user by the given emoji' do
-        merge_request3 = create(:merge_request, :simple, author: user, assignee: user, source_project: project2, target_project: project2, source_branch: 'other-branch')
+        merge_request3 = create(:merge_request, :simple, author: user, assignees: [user], source_project: project2, target_project: project2, source_branch: 'other-branch')
         award_emoji = create(:award_emoji, awardable: merge_request3, user: user2, name: 'star')
 
         get api('/merge_requests', user2), params: { my_reaction_emoji: award_emoji.name, scope: 'all' }
@@ -700,7 +704,7 @@ describe API::MergeRequests do
         get api("/projects/#{project.id}/merge_requests", user)
       end.count
 
-      create(:merge_request, author: user, assignee: user, source_project: project, target_project: project, created_at: base_time)
+      create(:merge_request, author: user, assignees: [user], source_project: project, target_project: project, created_at: base_time)
 
       expect do
         get api("/projects/#{project.id}/merge_requests", user)
@@ -730,7 +734,7 @@ describe API::MergeRequests do
 
   describe "GET /projects/:id/merge_requests/:merge_request_iid" do
     it 'matches json schema' do
-      merge_request = create(:merge_request, :with_test_reports, milestone: milestone1, author: user, assignee: user, source_project: project, target_project: project, title: "Test", created_at: base_time)
+      merge_request = create(:merge_request, :with_test_reports, milestone: milestone1, author: user, assignees: [user], source_project: project, target_project: project, title: "Test", created_at: base_time)
       get api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user)
 
       expect(response).to have_gitlab_http_status(200)
@@ -851,7 +855,7 @@ describe API::MergeRequests do
     end
 
     context 'Work in Progress' do
-      let!(:merge_request_wip) { create(:merge_request, author: user, assignee: user, source_project: project, target_project: project, title: "WIP: Test", created_at: base_time + 1.second) }
+      let!(:merge_request_wip) { create(:merge_request, author: user, assignees: [user], source_project: project, target_project: project, title: "WIP: Test", created_at: base_time + 1.second) }
 
       it "returns merge request" do
         get api("/projects/#{project.id}/merge_requests/#{merge_request_wip.iid}", user)
@@ -867,7 +871,7 @@ describe API::MergeRequests do
 
         merge_request_overflow = create(:merge_request, :simple,
                                         author: user,
-                                        assignee: user,
+                                        assignees: [user],
                                         source_project: project,
                                         source_branch: 'expand-collapse-files',
                                         target_project: project,
@@ -1005,6 +1009,71 @@ describe API::MergeRequests do
   end
 
   describe 'POST /projects/:id/merge_requests' do
+    context 'support for deprecated assignee_id' do
+      let(:params) do
+        {
+          title: 'Test merge request',
+          source_branch: 'feature_conflict',
+          target_branch: 'master',
+          author_id: user.id,
+          assignee_id: user2.id
+        }
+      end
+
+      it 'creates a new merge request' do
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignee']['name']).to eq(user2.name)
+        expect(json_response['assignees'].first['name']).to eq(user2.name)
+      end
+
+      it 'creates a new merge request when assignee_id is empty' do
+        params[:assignee_id] = ''
+
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignee']).to be_nil
+      end
+
+      it 'filters assignee_id of unauthorized user' do
+        private_project = create(:project, :private, :repository)
+        another_user = create(:user)
+        private_project.add_maintainer(user)
+        params[:assignee_id] = another_user.id
+
+        post api("/projects/#{private_project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['assignee']).to be_nil
+      end
+    end
+
+    context 'single assignee restrictions' do
+      let(:params) do
+        {
+          title: 'Test merge request',
+          source_branch: 'feature_conflict',
+          target_branch: 'master',
+          author_id: user.id,
+          assignee_ids: [user.id, user2.id]
+        }
+      end
+
+      it 'creates a new project merge request with no more than one assignee' do
+        post api("/projects/#{project.id}/merge_requests", user), params: params
+
+        expect(response).to have_gitlab_http_status(201)
+        expect(json_response['title']).to eq('Test merge request')
+        expect(json_response['assignees'].count).to eq(1)
+        expect(json_response['assignees'].first['name']).to eq(user.name)
+        expect(json_response.dig('assignee', 'name')).to eq(user.name)
+      end
+    end
+
     context 'between branches projects' do
       context 'different labels' do
         let(:params) do
@@ -1574,6 +1643,19 @@ describe API::MergeRequests do
       expect(json_response['force_remove_source_branch']).to be_truthy
     end
 
+    it 'filters assignee_id of unauthorized user' do
+      private_project = create(:project, :private, :repository)
+      mr = create(:merge_request, source_project: private_project, target_project: private_project)
+      another_user = create(:user)
+      private_project.add_maintainer(user)
+      params = { assignee_id: another_user.id }
+
+      put api("/projects/#{private_project.id}/merge_requests/#{mr.iid}", user), params: params
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(json_response['assignee']).to be_nil
+    end
+
     context 'when updating labels' do
       it 'allows special label names' do
         put api("/projects/#{project.id}/merge_requests/#{merge_request.iid}", user),
@@ -1728,7 +1810,7 @@ describe API::MergeRequests do
       issue = create(:issue, project: jira_project)
       description = "Closes #{ext_issue.to_reference(jira_project)}\ncloses #{issue.to_reference}"
       merge_request = create(:merge_request,
-        :simple, author: user, assignee: user, source_project: jira_project, description: description)
+        :simple, author: user, assignees: [user], source_project: jira_project, description: description)
 
       get api("/projects/#{jira_project.id}/merge_requests/#{merge_request.iid}/closes_issues", user)
 
diff --git a/spec/services/issuable/bulk_update_service_spec.rb b/spec/services/issuable/bulk_update_service_spec.rb
index ca366cdf1df..363b7266940 100644
--- a/spec/services/issuable/bulk_update_service_spec.rb
+++ b/spec/services/issuable/bulk_update_service_spec.rb
@@ -76,14 +76,14 @@ describe Issuable::BulkUpdateService do
   end
 
   describe 'updating merge request assignee' do
-    let(:merge_request) { create(:merge_request, target_project: project, source_project: project, assignee: user) }
+    let(:merge_request) { create(:merge_request, target_project: project, source_project: project, assignees: [user]) }
 
     context 'when the new assignee ID is a valid user' do
       it 'succeeds' do
         new_assignee = create(:user)
         project.add_developer(new_assignee)
 
-        result = bulk_update(merge_request, assignee_id: new_assignee.id)
+        result = bulk_update(merge_request, assignee_ids: [user.id, new_assignee.id])
 
         expect(result[:success]).to be_truthy
         expect(result[:count]).to eq(1)
@@ -93,22 +93,22 @@ describe Issuable::BulkUpdateService do
         assignee = create(:user)
         project.add_developer(assignee)
 
-        expect { bulk_update(merge_request, assignee_id: assignee.id) }
-          .to change { merge_request.reload.assignee }.from(user).to(assignee)
+        expect { bulk_update(merge_request, assignee_ids: [assignee.id]) }
+          .to change { merge_request.reload.assignee_ids }.from([user.id]).to([assignee.id])
       end
     end
 
     context "when the new assignee ID is #{IssuableFinder::NONE}" do
       it 'unassigns the issues' do
-        expect { bulk_update(merge_request, assignee_id: IssuableFinder::NONE) }
-          .to change { merge_request.reload.assignee }.to(nil)
+        expect { bulk_update(merge_request, assignee_ids: [IssuableFinder::NONE]) }
+          .to change { merge_request.reload.assignee_ids }.to([])
       end
     end
 
     context 'when the new assignee ID is not present' do
       it 'does not unassign' do
-        expect { bulk_update(merge_request, assignee_id: nil) }
-          .not_to change { merge_request.reload.assignee }
+        expect { bulk_update(merge_request, assignee_ids: []) }
+          .not_to change { merge_request.reload.assignee_ids }
       end
     end
   end
diff --git a/spec/services/issuable/destroy_service_spec.rb b/spec/services/issuable/destroy_service_spec.rb
index 8ccbba7fa58..15d1bb73ca3 100644
--- a/spec/services/issuable/destroy_service_spec.rb
+++ b/spec/services/issuable/destroy_service_spec.rb
@@ -34,7 +34,7 @@ describe Issuable::DestroyService do
     end
 
     context 'when issuable is a merge request' do
-      let!(:merge_request) { create(:merge_request, target_project: project, source_project: project, author: user, assignee: user) }
+      let!(:merge_request) { create(:merge_request, target_project: project, source_project: project, author: user, assignees: [user]) }
 
       it 'destroys the merge request' do
         expect { service.execute(merge_request) }.to change { project.merge_requests.count }.by(-1)
diff --git a/spec/services/members/destroy_service_spec.rb b/spec/services/members/destroy_service_spec.rb
index d37ca13ebd2..91bf4dccd77 100644
--- a/spec/services/members/destroy_service_spec.rb
+++ b/spec/services/members/destroy_service_spec.rb
@@ -43,9 +43,9 @@ describe Members::DestroyService do
   shared_examples 'a service destroying a member with access' do
     it_behaves_like 'a service destroying a member'
 
-    it 'invalidates cached counts for todos and assigned issues and merge requests', :aggregate_failures do
+    it 'invalidates cached counts for assigned issues and merge requests', :aggregate_failures do
       create(:issue, project: group_project, assignees: [member_user])
-      create(:merge_request, source_project: group_project, assignee: member_user)
+      create(:merge_request, source_project: group_project, assignees: [member_user])
       create(:todo, :pending, project: group_project, user: member_user)
       create(:todo, :done, project: group_project, user: member_user)
 
diff --git a/spec/services/merge_requests/close_service_spec.rb b/spec/services/merge_requests/close_service_spec.rb
index 433ffbd97f0..706bcea8199 100644
--- a/spec/services/merge_requests/close_service_spec.rb
+++ b/spec/services/merge_requests/close_service_spec.rb
@@ -4,7 +4,7 @@ describe MergeRequests::CloseService do
   let(:user) { create(:user) }
   let(:user2) { create(:user) }
   let(:guest) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignee: user2, author: create(:user)) }
+  let(:merge_request) { create(:merge_request, assignees: [user2], author: create(:user)) }
   let(:project) { merge_request.project }
   let!(:todo) { create(:todo, :assigned, user: user, project: project, target: merge_request, author: user2) }
 
diff --git a/spec/services/merge_requests/create_from_issue_service_spec.rb b/spec/services/merge_requests/create_from_issue_service_spec.rb
index 393299cce00..20bf1cbb8b6 100644
--- a/spec/services/merge_requests/create_from_issue_service_spec.rb
+++ b/spec/services/merge_requests/create_from_issue_service_spec.rb
@@ -118,7 +118,7 @@ describe MergeRequests::CreateFromIssueService do
 
       result = service.execute
 
-      expect(result[:merge_request].assignee).to eq(user)
+      expect(result[:merge_request].assignees).to eq([user])
     end
 
     context 'when ref branch is set' do
diff --git a/spec/services/merge_requests/create_service_spec.rb b/spec/services/merge_requests/create_service_spec.rb
index dc5d1cf2f04..30271e04c8e 100644
--- a/spec/services/merge_requests/create_service_spec.rb
+++ b/spec/services/merge_requests/create_service_spec.rb
@@ -32,7 +32,7 @@ describe MergeRequests::CreateService do
         expect(merge_request).to be_valid
         expect(merge_request.work_in_progress?).to be(false)
         expect(merge_request.title).to eq('Awesome merge_request')
-        expect(merge_request.assignee).to be_nil
+        expect(merge_request.assignees).to be_empty
         expect(merge_request.merge_params['force_remove_source_branch']).to eq('1')
       end
 
@@ -73,7 +73,7 @@ describe MergeRequests::CreateService do
               description: "well this is not done yet\n/wip",
               source_branch: 'feature',
               target_branch: 'master',
-              assignee: assignee
+              assignees: [assignee]
             }
           end
 
@@ -89,7 +89,7 @@ describe MergeRequests::CreateService do
               description: "well this is not done yet\n/wip",
               source_branch: 'feature',
               target_branch: 'master',
-              assignee: assignee
+              assignees: [assignee]
             }
           end
 
@@ -106,11 +106,11 @@ describe MergeRequests::CreateService do
             description: 'please fix',
             source_branch: 'feature',
             target_branch: 'master',
-            assignee: assignee
+            assignees: [assignee]
           }
         end
 
-        it { expect(merge_request.assignee).to eq assignee }
+        it { expect(merge_request.assignees).to eq([assignee]) }
 
         it 'creates a todo for new assignee' do
           attributes = {
@@ -301,7 +301,7 @@ describe MergeRequests::CreateService do
 
         let(:opts) do
           {
-            assignee_id: create(:user).id,
+            assignee_ids: create(:user).id,
             milestone_id: 1,
             title: 'Title',
             description: %(/assign @#{assignee.username}\n/milestone %"#{milestone.name}"),
@@ -317,7 +317,7 @@ describe MergeRequests::CreateService do
 
         it 'assigns and sets milestone to issuable from command' do
           expect(merge_request).to be_persisted
-          expect(merge_request.assignee).to eq(assignee)
+          expect(merge_request.assignees).to eq([assignee])
           expect(merge_request.milestone).to eq(milestone)
         end
       end
@@ -332,28 +332,28 @@ describe MergeRequests::CreateService do
         end
 
         it 'removes assignee_id when user id is invalid' do
-          opts = { title: 'Title', description: 'Description', assignee_id: -1 }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [-1] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee_id).to be_nil
+          expect(merge_request.assignee_ids).to be_empty
         end
 
         it 'removes assignee_id when user id is 0' do
-          opts = { title: 'Title', description: 'Description', assignee_id: 0 }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [0] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee_id).to be_nil
+          expect(merge_request.assignee_ids).to be_empty
         end
 
         it 'saves assignee when user id is valid' do
           project.add_maintainer(assignee)
-          opts = { title: 'Title', description: 'Description', assignee_id: assignee.id }
+          opts = { title: 'Title', description: 'Description', assignee_ids: [assignee.id] }
 
           merge_request = described_class.new(project, user, opts).execute
 
-          expect(merge_request.assignee).to eq(assignee)
+          expect(merge_request.assignees).to eq([assignee])
         end
 
         context 'when assignee is set' do
@@ -361,7 +361,7 @@ describe MergeRequests::CreateService do
             {
               title: 'Title',
               description: 'Description',
-              assignee_id: assignee.id,
+              assignee_ids: [assignee.id],
               source_branch: 'feature',
               target_branch: 'master'
             }
@@ -387,7 +387,7 @@ describe MergeRequests::CreateService do
           levels.each do |level|
             it "removes not authorized assignee when project is #{Gitlab::VisibilityLevel.level_name(level)}" do
               project.update(visibility_level: level)
-              opts = { title: 'Title', description: 'Description', assignee_id: assignee.id }
+              opts = { title: 'Title', description: 'Description', assignee_ids: [assignee.id] }
 
               merge_request = described_class.new(project, user, opts).execute
 
diff --git a/spec/services/merge_requests/ff_merge_service_spec.rb b/spec/services/merge_requests/ff_merge_service_spec.rb
index 1430e12a07e..a87d8b8752c 100644
--- a/spec/services/merge_requests/ff_merge_service_spec.rb
+++ b/spec/services/merge_requests/ff_merge_service_spec.rb
@@ -7,7 +7,7 @@ describe MergeRequests::FfMergeService do
     create(:merge_request,
            source_branch: 'flatten-dir',
            target_branch: 'improve/awesome',
-           assignee: user2,
+           assignees: [user2],
            author: create(:user))
   end
   let(:project) { merge_request.project }
diff --git a/spec/services/merge_requests/merge_service_spec.rb b/spec/services/merge_requests/merge_service_spec.rb
index 887ec17171e..b0b3273e3dc 100644
--- a/spec/services/merge_requests/merge_service_spec.rb
+++ b/spec/services/merge_requests/merge_service_spec.rb
@@ -3,7 +3,7 @@ require 'spec_helper'
 describe MergeRequests::MergeService do
   set(:user) { create(:user) }
   set(:user2) { create(:user) }
-  let(:merge_request) { create(:merge_request, :simple, author: user2, assignee: user2) }
+  let(:merge_request) { create(:merge_request, :simple, author: user2, assignees: [user2]) }
   let(:project) { merge_request.project }
 
   before do
@@ -111,7 +111,7 @@ describe MergeRequests::MergeService do
     end
 
     context 'closes related todos' do
-      let(:merge_request) { create(:merge_request, assignee: user, author: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user], author: user) }
       let(:project) { merge_request.project }
       let(:service) { described_class.new(project, user, commit_message: 'Awesome message') }
       let!(:todo) do
diff --git a/spec/services/merge_requests/merge_to_ref_service_spec.rb b/spec/services/merge_requests/merge_to_ref_service_spec.rb
index a3b48abae26..24d09c1fd00 100644
--- a/spec/services/merge_requests/merge_to_ref_service_spec.rb
+++ b/spec/services/merge_requests/merge_to_ref_service_spec.rb
@@ -149,7 +149,7 @@ describe MergeRequests::MergeToRefService do
     end
 
     context 'does not close related todos' do
-      let(:merge_request) { create(:merge_request, assignee: user, author: user) }
+      let(:merge_request) { create(:merge_request, assignees: [user], author: user) }
       let(:project) { merge_request.project }
       let!(:todo) do
         create(:todo, :assigned,
diff --git a/spec/services/merge_requests/post_merge_service_spec.rb b/spec/services/merge_requests/post_merge_service_spec.rb
index 5ad6f5528f9..2cebefee5d6 100644
--- a/spec/services/merge_requests/post_merge_service_spec.rb
+++ b/spec/services/merge_requests/post_merge_service_spec.rb
@@ -2,7 +2,7 @@ require 'spec_helper'
 
 describe MergeRequests::PostMergeService do
   let(:user) { create(:user) }
-  let(:merge_request) { create(:merge_request, assignee: user) }
+  let(:merge_request) { create(:merge_request, assignees: [user]) }
   let(:project) { merge_request.project }
 
   before do
diff --git a/spec/services/merge_requests/reopen_service_spec.rb b/spec/services/merge_requests/reopen_service_spec.rb
index 21e71509ed6..8b6db1ce33e 100644
--- a/spec/services/merge_requests/reopen_service_spec.rb
+++ b/spec/services/merge_requests/reopen_service_spec.rb
@@ -4,7 +4,7 @@ describe MergeRequests::ReopenService do
   let(:user) { create(:user) }
   let(:user2) { create(:user) }
   let(:guest) { create(:user) }
-  let(:merge_request) { create(:merge_request, :closed, assignee: user2, author: create(:user)) }
+  let(:merge_request) { create(:merge_request, :closed, assignees: [user2], author: create(:user)) }
   let(:project) { merge_request.project }
 
   before do
diff --git a/spec/services/merge_requests/update_service_spec.rb b/spec/services/merge_requests/update_service_spec.rb
index 8e367db031c..0525899ebfa 100644
--- a/spec/services/merge_requests/update_service_spec.rb
+++ b/spec/services/merge_requests/update_service_spec.rb
@@ -13,7 +13,7 @@ describe MergeRequests::UpdateService, :mailer do
   let(:merge_request) do
     create(:merge_request, :simple, title: 'Old title',
                                     description: "FYI #{user2.to_reference}",
-                                    assignee_id: user3.id,
+                                    assignee_ids: [user3.id],
                                     source_project: project,
                                     author: create(:user))
   end
@@ -48,7 +48,7 @@ describe MergeRequests::UpdateService, :mailer do
         {
           title: 'New title',
           description: 'Also please fix',
-          assignee_id: user2.id,
+          assignee_ids: [user.id],
           state_event: 'close',
           label_ids: [label.id],
           target_branch: 'target',
@@ -71,7 +71,7 @@ describe MergeRequests::UpdateService, :mailer do
       it 'matches base expectations' do
         expect(@merge_request).to be_valid
         expect(@merge_request.title).to eq('New title')
-        expect(@merge_request.assignee).to eq(user2)
+        expect(@merge_request.assignees).to match_array([user])
         expect(@merge_request).to be_closed
         expect(@merge_request.labels.count).to eq(1)
         expect(@merge_request.labels.first.title).to eq(label.name)
@@ -106,7 +106,7 @@ describe MergeRequests::UpdateService, :mailer do
         note = find_note('assigned to')
 
         expect(note).not_to be_nil
-        expect(note.note).to include "assigned to #{user2.to_reference}"
+        expect(note.note).to include "assigned to #{user.to_reference} and unassigned #{user3.to_reference}"
       end
 
       it 'creates a resource label event' do
@@ -293,7 +293,7 @@ describe MergeRequests::UpdateService, :mailer do
 
       context 'when is reassigned' do
         before do
-          update_merge_request({ assignee: user2 })
+          update_merge_request({ assignee_ids: [user2.id] })
         end
 
         it 'marks previous assignee pending todos as done' do
@@ -387,7 +387,7 @@ describe MergeRequests::UpdateService, :mailer do
 
       context 'when the assignee changes' do
         it 'updates open merge request counter for assignees when merge request is reassigned' do
-          update_merge_request(assignee_id: user2.id)
+          update_merge_request(assignee_ids: [user2.id])
 
           expect(user3.assigned_open_merge_requests_count).to eq 0
           expect(user2.assigned_open_merge_requests_count).to eq 1
@@ -541,36 +541,36 @@ describe MergeRequests::UpdateService, :mailer do
       end
     end
 
-    context 'updating asssignee_id' do
+    context 'updating asssignee_ids' do
       it 'does not update assignee when assignee_id is invalid' do
-        merge_request.update(assignee_id: user.id)
+        merge_request.update(assignee_ids: [user.id])
 
-        update_merge_request(assignee_id: -1)
+        update_merge_request(assignee_ids: [-1])
 
-        expect(merge_request.reload.assignee).to eq(user)
+        expect(merge_request.reload.assignees).to eq([user])
       end
 
       it 'unassigns assignee when user id is 0' do
-        merge_request.update(assignee_id: user.id)
+        merge_request.update(assignee_ids: [user.id])
 
-        update_merge_request(assignee_id: 0)
+        update_merge_request(assignee_ids: [0])
 
-        expect(merge_request.assignee_id).to be_nil
+        expect(merge_request.assignee_ids).to be_empty
       end
 
       it 'saves assignee when user id is valid' do
-        update_merge_request(assignee_id: user.id)
+        update_merge_request(assignee_ids: [user.id])
 
-        expect(merge_request.assignee_id).to eq(user.id)
+        expect(merge_request.assignee_ids).to eq([user.id])
       end
 
       it 'does not update assignee_id when user cannot read issue' do
-        non_member        = create(:user)
-        original_assignee = merge_request.assignee
+        non_member = create(:user)
+        original_assignees = merge_request.assignees
 
-        update_merge_request(assignee_id: non_member.id)
+        update_merge_request(assignee_ids: [non_member.id])
 
-        expect(merge_request.assignee_id).to eq(original_assignee.id)
+        expect(merge_request.reload.assignees).to eq(original_assignees)
       end
 
       context "when issuable feature is private" do
@@ -583,7 +583,7 @@ describe MergeRequests::UpdateService, :mailer do
             feature_visibility_attr = :"#{merge_request.model_name.plural}_access_level"
             project.project_feature.update_attribute(feature_visibility_attr, ProjectFeature::PRIVATE)
 
-            expect { update_merge_request(assignee_id: assignee) }.not_to change { merge_request.assignee }
+            expect { update_merge_request(assignee_ids: [assignee]) }.not_to change { merge_request.reload.assignees }
           end
         end
       end
@@ -619,7 +619,7 @@ describe MergeRequests::UpdateService, :mailer do
       end
 
       it 'is allowed by a user that can push to the source and can update the merge request' do
-        merge_request.update!(assignee: user)
+        merge_request.update!(assignees: [user])
         source_project.add_developer(user)
 
         update_merge_request(allow_collaboration: true, title: 'Updated title')
diff --git a/spec/services/notification_service_spec.rb b/spec/services/notification_service_spec.rb
index 9ba4a11104a..14c73852e65 100644
--- a/spec/services/notification_service_spec.rb
+++ b/spec/services/notification_service_spec.rb
@@ -125,11 +125,7 @@ describe NotificationService, :mailer do
 
   shared_examples 'participating by assignee notification' do
     it 'emails the participant' do
-      if issuable.is_a?(Issue)
-        issuable.assignees << participant
-      else
-        issuable.update_attribute(:assignee, participant)
-      end
+      issuable.assignees << participant
 
       notification_trigger
 
@@ -620,13 +616,13 @@ describe NotificationService, :mailer do
     context "merge request diff note" do
       let(:project) { create(:project, :repository) }
       let(:user) { create(:user) }
-      let(:merge_request) { create(:merge_request, source_project: project, assignee: user, author: create(:user)) }
+      let(:merge_request) { create(:merge_request, source_project: project, assignees: [user], author: create(:user)) }
       let(:note) { create(:diff_note_on_merge_request, project: project, noteable: merge_request) }
 
       before do
         build_team(note.project)
         project.add_maintainer(merge_request.author)
-        project.add_maintainer(merge_request.assignee)
+        merge_request.assignees.each { |assignee| project.add_maintainer(assignee) }
       end
 
       describe '#new_note' do
@@ -637,7 +633,7 @@ describe NotificationService, :mailer do
           notification.new_note(note)
 
           expect(SentNotification.last(3).map(&:recipient).map(&:id))
-            .to contain_exactly(merge_request.assignee.id, merge_request.author.id, @u_watcher.id)
+            .to contain_exactly(*merge_request.assignees.pluck(:id), merge_request.author.id, @u_watcher.id)
           expect(SentNotification.last.in_reply_to_discussion_id).to eq(note.discussion_id)
         end
       end
@@ -1223,11 +1219,12 @@ describe NotificationService, :mailer do
     let(:group) { create(:group) }
     let(:project) { create(:project, :public, :repository, namespace: group) }
     let(:another_project) { create(:project, :public, namespace: group) }
-    let(:merge_request) { create :merge_request, source_project: project, assignee: create(:user), description: 'cc @participant' }
+    let(:assignee) { create(:user) }
+    let(:merge_request) { create :merge_request, source_project: project, assignees: [assignee], description: 'cc @participant' }
 
     before do
       project.add_maintainer(merge_request.author)
-      project.add_maintainer(merge_request.assignee)
+      merge_request.assignees.each { |assignee| project.add_maintainer(assignee) }
       build_team(merge_request.target_project)
       add_users_with_subscription(merge_request.target_project, merge_request)
       update_custom_notification(:new_merge_request, @u_guest_custom, resource: project)
@@ -1239,7 +1236,7 @@ describe NotificationService, :mailer do
       it do
         notification.new_merge_request(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@watcher_and_subscriber)
         should_email(@u_participant_mentioned)
@@ -1254,9 +1251,11 @@ describe NotificationService, :mailer do
       it 'adds "assigned" reason for assignee, if any' do
         notification.new_merge_request(merge_request, @u_disabled)
 
-        email = find_email_for(merge_request.assignee)
+        merge_request.assignees.each do |assignee|
+          email = find_email_for(assignee)
 
-        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+          expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+        end
       end
 
       it "emails any mentioned users with the mention level" do
@@ -1347,9 +1346,9 @@ describe NotificationService, :mailer do
       end
 
       it do
-        notification.reassigned_merge_request(merge_request, current_user, merge_request.author)
+        notification.reassigned_merge_request(merge_request, current_user, [assignee])
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(merge_request.author)
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
@@ -1365,17 +1364,19 @@ describe NotificationService, :mailer do
       end
 
       it 'adds "assigned" reason for new assignee' do
-        notification.reassigned_merge_request(merge_request, current_user, merge_request.author)
+        notification.reassigned_merge_request(merge_request, current_user, [assignee])
 
-        email = find_email_for(merge_request.assignee)
+        merge_request.assignees.each do |assignee|
+          email = find_email_for(assignee)
 
-        expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+          expect(email).to have_header('X-GitLab-NotificationReason', NotificationReason::ASSIGNED)
+        end
       end
 
       it_behaves_like 'participating notifications' do
         let(:participant) { create(:user, username: 'user-participant') }
         let(:issuable) { merge_request }
-        let(:notification_trigger) { notification.reassigned_merge_request(merge_request, current_user, merge_request.author) }
+        let(:notification_trigger) { notification.reassigned_merge_request(merge_request, current_user, [assignee]) }
       end
     end
 
@@ -1388,7 +1389,7 @@ describe NotificationService, :mailer do
       it do
         notification.push_to_merge_request(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_guest_custom)
         should_email(@u_custom_global)
         should_email(@u_participant_mentioned)
@@ -1430,7 +1431,7 @@ describe NotificationService, :mailer do
         should_email(subscriber_1_to_group_label_2)
         should_email(subscriber_2_to_group_label_2)
         should_email(subscriber_to_label_2)
-        should_not_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_not_email(assignee) }
         should_not_email(merge_request.author)
         should_not_email(@u_watcher)
         should_not_email(@u_participant_mentioned)
@@ -1499,7 +1500,7 @@ describe NotificationService, :mailer do
       it do
         notification.close_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_guest_watcher)
         should_email(@u_guest_custom)
@@ -1529,7 +1530,7 @@ describe NotificationService, :mailer do
       it do
         notification.merge_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_guest_watcher)
         should_email(@u_guest_custom)
@@ -1581,7 +1582,7 @@ describe NotificationService, :mailer do
       it do
         notification.reopen_mr(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
         should_email(@subscriber)
@@ -1606,7 +1607,7 @@ describe NotificationService, :mailer do
       it do
         notification.resolve_all_discussions(merge_request, @u_disabled)
 
-        should_email(merge_request.assignee)
+        merge_request.assignees.each { |assignee| should_email(assignee) }
         should_email(@u_watcher)
         should_email(@u_participant_mentioned)
         should_email(@subscriber)
@@ -1850,8 +1851,8 @@ describe NotificationService, :mailer do
     let(:guest) { create(:user) }
     let(:developer) { create(:user) }
     let(:assignee) { create(:user) }
-    let(:merge_request) { create(:merge_request, source_project: private_project, assignee: assignee) }
-    let(:merge_request1) { create(:merge_request, source_project: private_project, assignee: assignee, description: "cc @#{guest.username}") }
+    let(:merge_request) { create(:merge_request, source_project: private_project, assignees: [assignee]) }
+    let(:merge_request1) { create(:merge_request, source_project: private_project, assignees: [assignee], description: "cc @#{guest.username}") }
     let(:note) { create(:note, noteable: merge_request, project: private_project) }
 
     before do
diff --git a/spec/services/quick_actions/interpret_service_spec.rb b/spec/services/quick_actions/interpret_service_spec.rb
index c7e5cca324f..c450f89c3cb 100644
--- a/spec/services/quick_actions/interpret_service_spec.rb
+++ b/spec/services/quick_actions/interpret_service_spec.rb
@@ -16,7 +16,9 @@ describe QuickActions::InterpretService do
   let(:service) { described_class.new(project, developer) }
 
   before do
-    stub_licensed_features(multiple_issue_assignees: false)
+    stub_licensed_features(multiple_issue_assignees: false,
+                           multiple_merge_request_assignees: false)
+
     project.add_developer(developer)
   end
 
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 13d7d795703..51c5a803dbd 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -166,8 +166,8 @@ describe SystemNoteService do
     end
   end
 
-  describe '.change_issue_assignees' do
-    subject { described_class.change_issue_assignees(noteable, project, author, [assignee]) }
+  describe '.change_issuable_assignees' do
+    subject { described_class.change_issuable_assignees(noteable, project, author, [assignee]) }
 
     let(:assignee) { create(:user) }
     let(:assignee1) { create(:user) }
@@ -180,7 +180,7 @@ describe SystemNoteService do
 
     def build_note(old_assignees, new_assignees)
       issue.assignees = new_assignees
-      described_class.change_issue_assignees(issue, project, author, old_assignees).note
+      described_class.change_issuable_assignees(issue, project, author, old_assignees).note
     end
 
     it_behaves_like 'a note with overridable created_at'
diff --git a/spec/services/todo_service_spec.rb b/spec/services/todo_service_spec.rb
index 8631f3f9a33..89411b2e908 100644
--- a/spec/services/todo_service_spec.rb
+++ b/spec/services/todo_service_spec.rb
@@ -272,28 +272,6 @@ describe TodoService do
       end
     end
 
-    describe '#reassigned_issue' do
-      it 'creates a pending todo for new assignee' do
-        unassigned_issue.assignees << john_doe
-        service.reassigned_issue(unassigned_issue, author)
-
-        should_create_todo(user: john_doe, target: unassigned_issue, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo if unassigned' do
-        issue.assignees.destroy_all # rubocop: disable DestroyAll
-
-        should_not_create_any_todo { service.reassigned_issue(issue, author) }
-      end
-
-      it 'creates a todo if new assignee is the current user' do
-        unassigned_issue.assignees << john_doe
-        service.reassigned_issue(unassigned_issue, john_doe)
-
-        should_create_todo(user: john_doe, target: unassigned_issue, author: john_doe, action: Todo::ASSIGNED)
-      end
-    end
-
     describe '#mark_pending_todos_as_done' do
       it 'marks related pending todos to the target for the user as done' do
         first_todo = create(:todo, :assigned, user: john_doe, project: project, target: issue, author: author)
@@ -504,10 +482,60 @@ describe TodoService do
     end
   end
 
+  describe '#reassigned_issuable' do
+    shared_examples 'reassigned issuable' do
+      it 'creates a pending todo for new assignee' do
+        issuable_unassigned.assignees = [john_doe]
+        service.reassigned_issuable(issuable_unassigned, author)
+
+        should_create_todo(user: john_doe, target: issuable_unassigned, action: Todo::ASSIGNED)
+      end
+
+      it 'does not create a todo if unassigned' do
+        issuable_assigned.assignees = []
+
+        should_not_create_any_todo { service.reassigned_issuable(issuable_assigned, author) }
+      end
+
+      it 'creates a todo if new assignee is the current user' do
+        issuable_assigned.assignees = [john_doe]
+        service.reassigned_issuable(issuable_assigned, john_doe)
+
+        should_create_todo(user: john_doe, target: issuable_assigned, author: john_doe, action: Todo::ASSIGNED)
+      end
+
+      it 'does not create a todo for guests' do
+        service.reassigned_issuable(issuable_assigned, author)
+        should_not_create_todo(user: guest, target: issuable_assigned, action: Todo::MENTIONED)
+      end
+
+      it 'does not create a directly addressed todo for guests' do
+        service.reassigned_issuable(addressed_issuable_assigned, author)
+        should_not_create_todo(user: guest, target: addressed_issuable_assigned, action: Todo::DIRECTLY_ADDRESSED)
+      end
+    end
+
+    context 'issuable is a merge request' do
+      it_behaves_like 'reassigned issuable' do
+        let(:issuable_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+        let(:addressed_issuable_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+        let(:issuable_unassigned) { create(:merge_request, source_project: project, author: author, assignees: []) }
+      end
+    end
+
+    context 'issuable is an issue' do
+      it_behaves_like 'reassigned issuable' do
+        let(:issuable_assigned) { create(:issue, project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+        let(:addressed_issuable_assigned) { create(:issue, project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+        let(:issuable_unassigned) { create(:issue, project: project, author: author, assignees: []) }
+      end
+    end
+  end
+
   describe 'Merge Requests' do
-    let(:mr_assigned) { create(:merge_request, source_project: project, author: author, assignee: john_doe, description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
-    let(:addressed_mr_assigned) { create(:merge_request, source_project: project, author: author, assignee: john_doe, description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
-    let(:mr_unassigned) { create(:merge_request, source_project: project, author: author, assignee: nil) }
+    let(:mr_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "- [ ] Task 1\n- [ ] Task 2 #{mentions}") }
+    let(:addressed_mr_assigned) { create(:merge_request, source_project: project, author: author, assignees: [john_doe], description: "#{directly_addressed}\n- [ ] Task 1\n- [ ] Task 2") }
+    let(:mr_unassigned) { create(:merge_request, source_project: project, author: author, assignees: []) }
 
     describe '#new_merge_request' do
       it 'creates a pending todo if assigned' do
@@ -659,38 +687,6 @@ describe TodoService do
       end
     end
 
-    describe '#reassigned_merge_request' do
-      it 'creates a pending todo for new assignee' do
-        mr_unassigned.update_attribute(:assignee, john_doe)
-        service.reassigned_merge_request(mr_unassigned, author)
-
-        should_create_todo(user: john_doe, target: mr_unassigned, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo if unassigned' do
-        mr_assigned.update_attribute(:assignee, nil)
-
-        should_not_create_any_todo { service.reassigned_merge_request(mr_assigned, author) }
-      end
-
-      it 'creates a todo if new assignee is the current user' do
-        mr_assigned.update_attribute(:assignee, john_doe)
-        service.reassigned_merge_request(mr_assigned, john_doe)
-
-        should_create_todo(user: john_doe, target: mr_assigned, author: john_doe, action: Todo::ASSIGNED)
-      end
-
-      it 'does not create a todo for guests' do
-        service.reassigned_merge_request(mr_assigned, author)
-        should_not_create_todo(user: guest, target: mr_assigned, action: Todo::MENTIONED)
-      end
-
-      it 'does not create a directly addressed todo for guests' do
-        service.reassigned_merge_request(addressed_mr_assigned, author)
-        should_not_create_todo(user: guest, target: addressed_mr_assigned, action: Todo::DIRECTLY_ADDRESSED)
-      end
-    end
-
     describe '#merge_merge_request' do
       it 'marks related pending todos to the target for the user as done' do
         first_todo = create(:todo, :assigned, user: john_doe, project: project, target: mr_assigned, author: author)
diff --git a/spec/services/users/destroy_service_spec.rb b/spec/services/users/destroy_service_spec.rb
index 83f1495a1c6..450e76d5f58 100644
--- a/spec/services/users/destroy_service_spec.rb
+++ b/spec/services/users/destroy_service_spec.rb
@@ -78,7 +78,7 @@ describe Users::DestroyService do
       end
 
       context "for an merge request the user was assigned to" do
-        let!(:merge_request) { create(:merge_request, source_project: project, assignee: user) }
+        let!(:merge_request) { create(:merge_request, source_project: project, assignees: [user]) }
 
         before do
           service.execute(user)
@@ -91,7 +91,7 @@ describe Users::DestroyService do
         it 'migrates the merge request so that it is "Unassigned"' do
           migrated_merge_request = MergeRequest.find_by_id(merge_request.id)
 
-          expect(migrated_merge_request.assignee).to be_nil
+          expect(migrated_merge_request.assignees).to be_empty
         end
       end
     end
diff --git a/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb b/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
index 4df80b4168a..ab6687f1d07 100644
--- a/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
+++ b/spec/support/shared_contexts/finders/merge_requests_finder_shared_contexts.rb
@@ -46,9 +46,9 @@ RSpec.shared_context 'MergeRequestsFinder multiple projects with merge requests
     allow_gitaly_n_plus_1 { create(:project, group: subgroup) }
   end
 
-  let!(:merge_request1) { create(:merge_request, author: user, source_project: project2, target_project: project1, target_branch: 'merged-target') }
-  let!(:merge_request2) { create(:merge_request, :conflict, author: user, source_project: project2, target_project: project1, state: 'closed') }
-  let!(:merge_request3) { create(:merge_request, :simple, author: user, source_project: project2, target_project: project2, state: 'locked', title: 'thing WIP thing') }
+  let!(:merge_request1) { create(:merge_request, assignees: [user], author: user, source_project: project2, target_project: project1, target_branch: 'merged-target') }
+  let!(:merge_request2) { create(:merge_request, :conflict, assignees: [user], author: user, source_project: project2, target_project: project1, state: 'closed') }
+  let!(:merge_request3) { create(:merge_request, :simple, author: user, assignees: [user2], source_project: project2, target_project: project2, state: 'locked', title: 'thing WIP thing') }
   let!(:merge_request4) { create(:merge_request, :simple, author: user, source_project: project3, target_project: project3, title: 'WIP thing') }
   let!(:merge_request5) { create(:merge_request, :simple, author: user, source_project: project4, target_project: project4, title: '[WIP]') }
 
diff --git a/spec/support/shared_contexts/merge_request_create.rb b/spec/support/shared_contexts/merge_request_create.rb
new file mode 100644
index 00000000000..529f481c2b6
--- /dev/null
+++ b/spec/support/shared_contexts/merge_request_create.rb
@@ -0,0 +1,26 @@
+# frozen_string_literal: true
+
+shared_context 'merge request create context' do
+  let(:user)        { create(:user) }
+  let(:user2)       { create(:user) }
+  let(:target_project) { create(:project, :public, :repository) }
+  let(:source_project) { target_project }
+  let!(:milestone)   { create(:milestone, project: target_project) }
+  let!(:label)       { create(:label, project: target_project) }
+  let!(:label2)      { create(:label, project: target_project) }
+
+  before do
+    source_project.add_maintainer(user)
+    target_project.add_maintainer(user)
+    target_project.add_maintainer(user2)
+
+    sign_in(user)
+    visit project_new_merge_request_path(target_project,
+                                         merge_request: {
+                                           source_project_id: source_project.id,
+                                           target_project_id: target_project.id,
+                                           source_branch: 'fix',
+                                           target_branch: 'master'
+                                         })
+  end
+end
diff --git a/spec/support/shared_contexts/merge_request_edit.rb b/spec/support/shared_contexts/merge_request_edit.rb
new file mode 100644
index 00000000000..c84510ff47d
--- /dev/null
+++ b/spec/support/shared_contexts/merge_request_edit.rb
@@ -0,0 +1,28 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+shared_context 'merge request edit context' do
+  let(:user)        { create(:user) }
+  let(:user2)       { create(:user) }
+  let!(:milestone)   { create(:milestone, project: target_project) }
+  let!(:label)       { create(:label, project: target_project) }
+  let!(:label2)      { create(:label, project: target_project) }
+  let(:target_project) { create(:project, :public, :repository) }
+  let(:source_project) { target_project }
+  let(:merge_request) do
+    create(:merge_request,
+           source_project: source_project,
+           target_project: target_project,
+           source_branch: 'fix',
+           target_branch: 'master')
+  end
+
+  before do
+    source_project.add_maintainer(user)
+    target_project.add_maintainer(user)
+    target_project.add_maintainer(user2)
+
+    sign_in(user)
+    visit edit_project_merge_request_path(target_project, merge_request)
+  end
+end
diff --git a/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb b/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
index 7038a366144..ec1b1754cf0 100644
--- a/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
+++ b/spec/support/shared_examples/features/creatable_merge_request_shared_examples.rb
@@ -1,42 +1,17 @@
 RSpec.shared_examples 'a creatable merge request' do
   include WaitForRequests
 
-  let(:user)        { create(:user) }
-  let(:user2)       { create(:user) }
-  let(:target_project) { create(:project, :public, :repository) }
-  let(:source_project) { target_project }
-  let!(:milestone)   { create(:milestone, project: target_project) }
-  let!(:label)       { create(:label, project: target_project) }
-  let!(:label2)      { create(:label, project: target_project) }
-
-  before do
-    source_project.add_maintainer(user)
-    target_project.add_maintainer(user)
-    target_project.add_maintainer(user2)
-
-    sign_in(user)
-    visit project_new_merge_request_path(
-      target_project,
-      merge_request: {
-        source_project_id: source_project.id,
-        target_project_id: target_project.id,
-        source_branch: 'fix',
-        target_branch: 'master'
-      })
-  end
-
   it 'creates new merge request', :js do
-    click_button 'Assignee'
+    find('.js-assignee-search').click
     page.within '.dropdown-menu-user' do
       click_link user2.name
     end
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user2.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user2.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user2.name
     end
-
     click_link 'Assign to me'
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user.name
     end
diff --git a/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb b/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
index eef0327c9a6..a6121fcc50a 100644
--- a/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
+++ b/spec/support/shared_examples/features/editable_merge_request_shared_examples.rb
@@ -1,34 +1,10 @@
 RSpec.shared_examples 'an editable merge request' do
-  let(:user)        { create(:user) }
-  let(:user2)       { create(:user) }
-  let!(:milestone)   { create(:milestone, project: target_project) }
-  let!(:label)       { create(:label, project: target_project) }
-  let!(:label2)      { create(:label, project: target_project) }
-  let(:target_project) { create(:project, :public, :repository) }
-  let(:source_project) { target_project }
-  let(:merge_request) do
-    create(:merge_request,
-      source_project: source_project,
-      target_project: target_project,
-      source_branch: 'fix',
-      target_branch: 'master')
-  end
-
-  before do
-    source_project.add_maintainer(user)
-    target_project.add_maintainer(user)
-    target_project.add_maintainer(user2)
-
-    sign_in(user)
-    visit edit_project_merge_request_path(target_project, merge_request)
-  end
-
   it 'updates merge request', :js do
-    click_button 'Assignee'
+    find('.js-assignee-search').click
     page.within '.dropdown-menu-user' do
       click_link user.name
     end
-    expect(find('input[name="merge_request[assignee_id]"]', visible: false).value).to match(user.id.to_s)
+    expect(find('input[name="merge_request[assignee_ids][]"]', visible: false).value).to match(user.id.to_s)
     page.within '.js-assignee-search' do
       expect(page).to have_content user.name
     end
diff --git a/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb b/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
new file mode 100644
index 00000000000..bab7963f06f
--- /dev/null
+++ b/spec/support/shared_examples/features/multiple_assignees_mr_shared_examples.rb
@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+shared_examples 'multiple assignees merge request' do |action, save_button_title|
+  it "#{action} a MR with multiple assignees", :js do
+    find('.js-assignee-search').click
+    page.within '.dropdown-menu-user' do
+      click_link user.name
+      click_link user2.name
+    end
+
+    # Extra click needed in order to toggle the dropdown
+    find('.js-assignee-search').click
+
+    expect(all('input[name="merge_request[assignee_ids][]"]', visible: false).map(&:value))
+      .to match_array([user.id.to_s, user2.id.to_s])
+
+    page.within '.js-assignee-search' do
+      expect(page).to have_content "#{user2.name} + 1 more"
+    end
+
+    click_button save_button_title
+
+    page.within '.issuable-sidebar' do
+      page.within '.assignee' do
+        expect(page).to have_content '2 Assignees'
+
+        click_link 'Edit'
+
+        expect(page).to have_content user.name
+        expect(page).to have_content user2.name
+      end
+    end
+
+    page.within '.dropdown-menu-user' do
+      click_link user.name
+    end
+
+    page.within '.issuable-sidebar' do
+      page.within '.assignee' do
+        # Closing dropdown to persist
+        click_link 'Edit'
+
+        expect(page).to have_content user2.name
+      end
+    end
+  end
+end
diff --git a/spec/support/shared_examples/finders/assignees_filter_spec.rb b/spec/support/shared_examples/finders/assignees_filter_spec.rb
new file mode 100644
index 00000000000..782a2d97746
--- /dev/null
+++ b/spec/support/shared_examples/finders/assignees_filter_spec.rb
@@ -0,0 +1,49 @@
+# frozen_string_literal: true
+
+shared_examples 'assignee ID filter' do
+  it 'returns issuables assigned to that user' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'assignee username filter' do
+  it 'returns issuables assigned to those users' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'no assignee filter' do
+  let(:params) { { assignee_id: 'None' } }
+
+  it 'returns issuables not assigned to any assignee' do
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+
+  it 'returns issuables not assigned to any assignee' do
+    params[:assignee_id] = 0
+
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+
+  it 'returns issuables not assigned to any assignee' do
+    params[:assignee_id] = 'none'
+
+    expect(issuables).to contain_exactly(*expected_issuables)
+  end
+end
+
+shared_examples 'any assignee filter' do
+  context '' do
+    let(:params) { { assignee_id: 'Any' } }
+
+    it 'returns issuables assigned to any assignee' do
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+
+    it 'returns issuables assigned to any assignee' do
+      params[:assignee_id] = 'any'
+
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+  end
+end
diff --git a/spec/views/projects/merge_requests/edit.html.haml_spec.rb b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
index c13eab30054..529afa03f9c 100644
--- a/spec/views/projects/merge_requests/edit.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/edit.html.haml_spec.rb
@@ -17,7 +17,7 @@ describe 'projects/merge_requests/edit.html.haml' do
       source_project: forked_project,
       target_project: project,
       author: user,
-      assignee: user,
+      assignees: [user],
       milestone: milestone)
   end
 
@@ -40,7 +40,7 @@ describe 'projects/merge_requests/edit.html.haml' do
 
       expect(rendered).to have_field('merge_request[title]')
       expect(rendered).to have_field('merge_request[description]')
-      expect(rendered).to have_selector('#merge_request_assignee_id', visible: false)
+      expect(rendered).to have_selector('input[name="merge_request[label_ids][]"]', visible: false)
       expect(rendered).to have_selector('#merge_request_milestone_id', visible: false)
       expect(rendered).not_to have_selector('#merge_request_target_branch', visible: false)
     end
@@ -52,7 +52,7 @@ describe 'projects/merge_requests/edit.html.haml' do
 
       expect(rendered).to have_field('merge_request[title]')
       expect(rendered).to have_field('merge_request[description]')
-      expect(rendered).to have_selector('#merge_request_assignee_id', visible: false)
+      expect(rendered).to have_selector('input[name="merge_request[label_ids][]"]', visible: false)
       expect(rendered).to have_selector('#merge_request_milestone_id', visible: false)
       expect(rendered).to have_selector('#merge_request_target_branch', visible: false)
     end
diff --git a/spec/views/projects/merge_requests/show.html.haml_spec.rb b/spec/views/projects/merge_requests/show.html.haml_spec.rb
index d9bda1a3414..23cb319a202 100644
--- a/spec/views/projects/merge_requests/show.html.haml_spec.rb
+++ b/spec/views/projects/merge_requests/show.html.haml_spec.rb
@@ -53,19 +53,6 @@ describe 'projects/merge_requests/show.html.haml' do
         expect(rendered).not_to have_css('.cannot-be-merged')
       end
     end
-
-    context 'when assignee is not allowed to merge' do
-      it 'shows a warning icon' do
-        reporter = create(:user)
-        project.add_reporter(reporter)
-        closed_merge_request.update(assignee_id: reporter.id)
-        assign(:issuable_sidebar, serialize_issuable_sidebar(user, project, closed_merge_request))
-
-        render
-
-        expect(rendered).to have_css('.cannot-be-merged')
-      end
-    end
   end
 
   context 'when the merge request is closed' do