Preload user notes count in GraphQL

app/graphql/types/merge_request_type.rb

@@ -159,6 +159,18 @@ module Types
       object.approved_by_users
     end
 
+    # rubocop: disable CodeReuse/ActiveRecord
+    def user_notes_count
+      BatchLoader::GraphQL.for(object.id).batch(key: :merge_request_user_notes_count) do |ids, loader, args|
+        counts = Note.where(noteable_type: 'MergeRequest', noteable_id: ids).user.group(:noteable_id).count
+
+        ids.each do |id|
+          loader.call(id, counts[id] || 0)
+        end
+      end
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
     def diff_stats(path: nil)
       stats = Array.wrap(object.diff_stats&.to_a)
 

changelogs/unreleased/batch-load-user-notes-count.yml

+---
+title: Preload `user_notes_count` in MergeRequest GraphQL API
+merge_request: 44894
+author:
+type: performance

spec/requests/api/graphql/project/merge_requests_spec.rb

@@ -232,7 +232,19 @@ RSpec.describe 'getting merge request listings nested in a project' do
 
       include_examples 'N+1 query check'
     end
+
+    context 'when requesting `user_notes_count`' do
+      let(:requested_fields) { [:user_notes_count] }
+
+      before do
+        create_list(:note_on_merge_request, 2, noteable: merge_request_a, project: project)
+        create(:note_on_merge_request, noteable: merge_request_c, project: project)
+      end
+
+      include_examples 'N+1 query check'
+    end
   end
+
   describe 'sorting and pagination' do
     let(:data_path) { [:project, :mergeRequests] }