Fix IDE detecting MR from fork branchide-fix-detect-mr-from-fork

**Why?**
Currently the IDE loads a merge request based on only the
`source_branch` name. This means it loads MR's from
forks that have the same branch name (not good).

- This required updating the BE API to accept `source_project_id`

6 files changed, 34 insertions, 5 deletions

diff --git a/app/assets/javascripts/ide/stores/actions/merge_request.js b/app/assets/javascripts/ide/stores/actions/merge_request.js
index 362ced248a1..1273e375859 100644
--- a/app/assets/javascripts/ide/stores/actions/merge_request.js
+++ b/app/assets/javascripts/ide/stores/actions/merge_request.js
@@ -4,10 +4,11 @@ import service from '../../services';
 import * as types from '../mutation_types';
 import { activityBarViews } from '../../constants';
 
-export const getMergeRequestsForBranch = ({ commit }, { projectId, branchId } = {}) =>
+export const getMergeRequestsForBranch = ({ commit, state }, { projectId, branchId } = {}) =>
   service
     .getProjectMergeRequests(`${projectId}`, {
       source_branch: branchId,
+      source_project_id: state.projects[projectId].id,
       order_by: 'created_at',
       per_page: 1,
     })
diff --git a/app/finders/merge_requests_finder.rb b/app/finders/merge_requests_finder.rb
index 84689ff5dc7..29947bc94d5 100644
--- a/app/finders/merge_requests_finder.rb
+++ b/app/finders/merge_requests_finder.rb
@@ -40,7 +40,8 @@ class MergeRequestsFinder < IssuableFinder
     items = by_commit(super)
     items = by_source_branch(items)
     items = by_wip(items)
-    by_target_branch(items)
+    items = by_target_branch(items)
+    by_source_project_id(items)
   end
 
   private
@@ -74,6 +75,16 @@ class MergeRequestsFinder < IssuableFinder
     items.where(target_branch: target_branch)
   end
 
+  def source_project_id
+    @source_project_id ||= params[:source_project_id].presence
+  end
+
+  def by_source_project_id(items)
+    return items unless source_project_id
+
+    items.where(source_project_id: source_project_id)
+  end
+
   def by_wip(items)
     if params[:wip] == 'yes'
       items.where(wip_match(items.arel_table))
diff --git a/changelogs/unreleased/ide-fix-detect-mr-from-fork.yml b/changelogs/unreleased/ide-fix-detect-mr-from-fork.yml
new file mode 100644
index 00000000000..8f4f49896d7
--- /dev/null
+++ b/changelogs/unreleased/ide-fix-detect-mr-from-fork.yml
@@ -0,0 +1,5 @@
+---
+title: Fix IDE detection of MR from fork with same branch name
+merge_request: 26986
+author:
+type: fixed
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 98dcc388f44..e4b21b7d1c4 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -111,6 +111,7 @@ module API
                          desc: 'Return merge requests for the given scope: `created_by_me`, `assigned_to_me` or `all`'
         optional :my_reaction_emoji, type: String, desc: 'Return issues reacted by the authenticated user by the given emoji'
         optional :source_branch, type: String, desc: 'Return merge requests with the given source branch'
+        optional :source_project_id, type: Integer, desc: 'Return merge requests with the given source project id'
         optional :target_branch, type: String, desc: 'Return merge requests with the given target branch'
         optional :search, type: String, desc: 'Search merge requests for text present in the title, description, or any combination of these'
         optional :in, type: String, desc: '`title`, `description`, or a string joining them with comma'
diff --git a/spec/finders/merge_requests_finder_spec.rb b/spec/finders/merge_requests_finder_spec.rb
index 56136eb84bc..f508b9bdb6f 100644
--- a/spec/finders/merge_requests_finder_spec.rb
+++ b/spec/finders/merge_requests_finder_spec.rb
@@ -83,6 +83,14 @@ describe MergeRequestsFinder do
         expect(merge_requests).to contain_exactly(merge_request2)
       end
 
+      it 'filters by source project id' do
+        params = { source_project_id: merge_request2.source_project_id }
+
+        merge_requests = described_class.new(user, params).execute
+
+        expect(merge_requests).to contain_exactly(merge_request1, merge_request2, merge_request3)
+      end
+
       it 'filters by state' do
         params = { state: 'locked' }
 
diff --git a/spec/javascripts/ide/stores/actions/merge_request_spec.js b/spec/javascripts/ide/stores/actions/merge_request_spec.js
index c2ad476d0aa..4dd0c1150eb 100644
--- a/spec/javascripts/ide/stores/actions/merge_request_spec.js
+++ b/spec/javascripts/ide/stores/actions/merge_request_spec.js
@@ -2,7 +2,6 @@ import MockAdapter from 'axios-mock-adapter';
 import axios from '~/lib/utils/axios_utils';
 import store from '~/ide/stores';
 import actions, {
-  getMergeRequestsForBranch,
   getMergeRequestData,
   getMergeRequestChanges,
   getMergeRequestVersions,
@@ -13,6 +12,7 @@ import { activityBarViews } from '~/ide/constants';
 import { resetStore } from '../../helpers';
 
 const TEST_PROJECT = 'abcproject';
+const TEST_PROJECT_ID = 17;
 
 describe('IDE store merge request actions', () => {
   let mock;
@@ -20,7 +20,8 @@ describe('IDE store merge request actions', () => {
   beforeEach(() => {
     mock = new MockAdapter(axios);
 
-    store.state.projects.abcproject = {
+    store.state.projects[TEST_PROJECT] = {
+      id: TEST_PROJECT_ID,
       mergeRequests: {},
     };
   });
@@ -47,6 +48,7 @@ describe('IDE store merge request actions', () => {
             .then(() => {
               expect(service.getProjectMergeRequests).toHaveBeenCalledWith(TEST_PROJECT, {
                 source_branch: 'bar',
+                source_project_id: TEST_PROJECT_ID,
                 order_by: 'created_at',
                 per_page: 1,
               });
@@ -106,7 +108,8 @@ describe('IDE store merge request actions', () => {
       it('flashes message, if error', done => {
         const flashSpy = spyOnDependency(actions, 'flash');
 
-        getMergeRequestsForBranch({ commit() {} }, { projectId: TEST_PROJECT, branchId: 'bar' })
+        store
+          .dispatch('getMergeRequestsForBranch', { projectId: TEST_PROJECT, branchId: 'bar' })
           .then(() => {
             fail('Expected getMergeRequestsForBranch to throw an error');
           })