Fixed issue with assignee object not being returned

Closes #15515

app/controllers/projects/issues_controller.rb

@@ -109,7 +109,7 @@ class Projects::IssuesController < Projects::ApplicationController
         end
       end
       format.json do
-        render json: @issue.to_json(include: [:milestone, labels: { methods: :text_color }, assignee: { methods: :avatar_url }])
+        render json: @issue.to_json(include: { milestone: {}, assignee: { methods: :avatar_url }, labels: { methods: :text_color } })
       end
     end
   end

app/controllers/projects/merge_requests_controller.rb

@@ -154,7 +154,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
                        @merge_request.target_project, @merge_request])
         end
         format.json do
-          render json: @merge_request.to_json(include: [:milestone, labels: { methods: :text_color }, assignee: { methods: :avatar_url }])
+          render json: @merge_request.to_json(include: { milestone: {}, assignee: { methods: :avatar_url }, labels: { methods: :text_color } })
         end
       end
     else

spec/features/issues_spec.rb

@@ -317,6 +317,27 @@ describe 'Issues', feature: true do
         expect(issue.reload.assignee).to be_nil
       end
+
+      it 'allows user to select an assignee', js: true do
+        issue2 = create(:issue, project: project, author: @user)
+        visit namespace_project_issue_path(project.namespace, project, issue2)
+
+        page.within('.assignee') do
+          expect(page).to have_content "No assignee"
+        end
+
+        page.within '.assignee' do
+          click_link 'Edit'
+        end
+        
+        page.within '.dropdown-menu-user' do
+          click_link @user.name
+        end
+
+        page.within('.assignee') do
+          expect(page).to have_content @user.name
+        end
+      end
     end
     context 'by unauthorized user' do