Fall back to the `MergeRequestWidgetEntity`

app/serializers/merge_request_serializer.rb

@@ -7,7 +7,7 @@ class MergeRequestSerializer < BaseSerializer
       case opts[:serializer]
       when 'basic', 'sidebar'
         MergeRequestBasicEntity
-      when 'widget'
+      else # It's 'widget'
         MergeRequestWidgetEntity
       end
 

spec/controllers/projects/merge_requests_controller_spec.rb

@@ -98,6 +98,14 @@ describe Projects::MergeRequestsController do
           expect(response).to match_response_schema('entities/merge_request_widget')
         end
       end
+
+      context 'when no serialiser was passed' do
+        it 'renders widget MR entity as json' do
+          go(serializer: nil, format: :json)
+
+          expect(response).to match_response_schema('entities/merge_request_widget')
+        end
+      end
     end
 
     describe "as diff" do

spec/serializers/merge_request_serializer_spec.rb

@@ -36,8 +36,8 @@ describe MergeRequestSerializer do
   context 'no serializer' do
     let(:serializer) { nil }
 
-    it 'raises an error' do
-      expect { json_entity }.to raise_error(NoMethodError)
+    it 'falls back to the widget entity' do
+      expect(json_entity).to match_schema('entities/merge_request_widget')
     end
   end
 end