Fix Error 500 when using closes_issues API with an external issue tracker

Closes #18484

3 files changed, 14 insertions, 1 deletions

diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 50d69274b2e..14370ac218d 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -179,6 +179,11 @@ module API
       expose :upvotes, :downvotes
     end
 
+    class ExternalIssue < Grape::Entity
+      expose :title
+      expose :id
+    end
+
     class MergeRequest < ProjectEntity
       expose :target_branch, :source_branch
       expose :upvotes, :downvotes
diff --git a/lib/api/helpers.rb b/lib/api/helpers.rb
index e1d3bbcc02d..de5959e3aae 100644
--- a/lib/api/helpers.rb
+++ b/lib/api/helpers.rb
@@ -418,5 +418,13 @@ module API
     def send_git_archive(repository, ref:, format:)
       header(*Gitlab::Workhorse.send_git_archive(repository, ref: ref, format: format))
     end
+
+    def issue_entity(project)
+      if project.has_external_issue_tracker?
+        Entities::ExternalIssue
+      else
+        Entities::Issue
+      end
+    end
   end
 end
diff --git a/lib/api/merge_requests.rb b/lib/api/merge_requests.rb
index 24df3e397e0..0e94efd4acd 100644
--- a/lib/api/merge_requests.rb
+++ b/lib/api/merge_requests.rb
@@ -329,7 +329,7 @@ module API
         get "#{path}/closes_issues" do
           merge_request = user_project.merge_requests.find(params[:merge_request_id])
           issues = ::Kaminari.paginate_array(merge_request.closes_issues(current_user))
-          present paginate(issues), with: Entities::Issue, current_user: current_user
+          present paginate(issues), with: issue_entity(user_project), current_user: current_user
         end
       end
     end