Directly use MRs given by ReferencedMergeRequestsService

lib/api/issues.rb

@@ -304,19 +304,14 @@ module API
       get ':id/issues/:issue_iid/related_merge_requests' do
         issue = find_project_issue(params[:issue_iid])
 
-        merge_request_iids = ::Issues::ReferencedMergeRequestsService.new(user_project, current_user)
+        merge_requests = ::Issues::ReferencedMergeRequestsService.new(user_project, current_user)
           .execute(issue)
           .flatten
-          .map(&:iid)
 
-        merge_requests =
-          if merge_request_iids.present?
-            MergeRequest.all.where(iid: merge_request_iids)
-          else
-            MergeRequest.none
-          end
-
-        present paginate(merge_requests), with: Entities::MergeRequestBasic, current_user: current_user, project: user_project
+        present paginate(::Kaminari.paginate_array(merge_requests)),
+          with: Entities::MergeRequestBasic,
+          current_user: current_user,
+          project: user_project
       end
 
       desc 'List merge requests closing issue' do

spec/requests/api/issues_spec.rb

@@ -1835,7 +1835,7 @@ describe API::Issues do
 
       get_related_merge_requests(project.id, issue.iid, user)
 
-      expect_paginated_array_response(merge_request.id)
+      expect_paginated_array_response([related_mr.id, merge_request.id])
     end
 
     context 'no merge request mentioned a issue' do