Expose positions and line_codes of a diff note

Currently it contains position of a note on merge-ref-head
In the future it may extend

app/models/diff_discussion.rb

@@ -13,6 +13,7 @@ class DiffDiscussion < Discussion
   delegate  :position,
             :original_position,
             :change_position,
+            :diff_note_positions,
             :on_text?,
             :on_image?,
 

app/models/note.rb

@@ -125,7 +125,7 @@ class Note < ApplicationRecord
   scope :inc_author, -> { includes(:author) }
   scope :inc_relations_for_view, -> do
     includes(:project, { author: :status }, :updated_by, :resolved_by, :award_emoji,
-             { system_note_metadata: :description_version }, :note_diff_file, :suggestions)
+             { system_note_metadata: :description_version }, :note_diff_file, :diff_note_positions, :suggestions)
   end
 
   scope :with_notes_filter, -> (notes_filter) do

app/serializers/discussion_entity.rb

@@ -20,6 +20,14 @@ class DiscussionEntity < Grape::Entity
     discussion_path(discussion)
   end
 
+  expose :positions, if: -> (d, _) { display_merge_ref_discussions?(d) } do |discussion|
+    discussion.diff_note_positions.map(&:position)
+  end
+
+  expose :line_codes, if: -> (d, _) { display_merge_ref_discussions?(d) } do |discussion|
+    discussion.diff_note_positions.map(&:line_code)
+  end
+
   expose :individual_note?, as: :individual_note
   expose :resolvable do |discussion|
     discussion.resolvable?
@@ -59,4 +67,11 @@ class DiscussionEntity < Grape::Entity
   def current_user
     request.current_user
   end
+
+  def display_merge_ref_discussions?(discussion)
+    return unless discussion.diff_discussion?
+    return if discussion.legacy_diff_discussion?
+
+    Feature.enabled?(:merge_ref_head_comments, discussion.project)
+  end
 end

spec/serializers/discussion_entity_spec.rb

@@ -73,9 +73,19 @@ describe DiscussionEntity do
         :diff_file,
         :truncated_diff_lines,
         :position,
+        :positions,
+        :line_codes,
         :line_code,
         :active
       )
     end
+
+    context 'diff_head_compare feature is disabled' do
+      it 'does not expose positions and line_codes attributes' do
+        stub_feature_flags(merge_ref_head_comments: false)
+
+        expect(subject.keys).not_to include(:positions, :line_codes)
+      end
+    end
   end
 end