Fix discussion entity for legacy diff notes

app/serializers/discussion_entity.rb

@@ -3,7 +3,7 @@ class DiscussionEntity < Grape::Entity
   include NotesHelper
 
   expose :id, :reply_id
-  expose :position, if: -> (d, _) { d.diff_discussion? }
+  expose :position, if: -> (d, _) { d.diff_discussion? && !d.legacy_diff_discussion?}
   expose :line_code, if: -> (d, _) { d.diff_discussion? }
   expose :expanded?, as: :expanded
   expose :active?, as: :active, if: -> (d, _) { d.diff_discussion? }

spec/serializers/discussion_entity_spec.rb

@@ -36,6 +36,25 @@
     )
   end
 
+  context 'when is LegacyDiffDiscussion' do
+    let(:project) { create(:project) }
+    let(:merge_request) { create(:merge_request, source_project: project) }
+    let(:discussion) { create(:legacy_diff_note_on_merge_request, noteable: merge_request, project: project).to_discussion }
+
+    it 'exposes correct attributes' do
+      expect(subject.keys.sort).to include(
+        :diff_discussion,
+        :expanded,
+        :id,
+        :individual_note,
+        :notes,
+        :discussion_path,
+        :for_commit,
+        :commit_id
+      )
+    end
+  end
+
   context 'when diff file is present' do
     let(:note) { create(:diff_note_on_merge_request) }