Fix CI pipelines N+1 in Issues::ReferencedMergeRequestsService

Whether the preloading belongs in the service or the controller is arguable,
here. As the service is only used for one controller action, it seems reasonable
to put it in the service, but that is not a definitive answer.

Adding the preloads for MR project routes here doesn't seem to work, perhaps
because of https://github.com/rails/rails/issues/32140.

1 files changed, 18 insertions, 0 deletions

diff --git a/spec/services/issues/referenced_merge_requests_service_spec.rb b/spec/services/issues/referenced_merge_requests_service_spec.rb
index 3349b97e688..d57bcf55af1 100644
--- a/spec/services/issues/referenced_merge_requests_service_spec.rb
+++ b/spec/services/issues/referenced_merge_requests_service_spec.rb
@@ -47,6 +47,24 @@ describe Issues::ReferencedMergeRequestsService do
 
         expect { service.execute(issue) }.not_to exceed_query_limit(control_count)
       end
+
+      it 'preloads the head pipeline for each merge request, and its routes' do
+        # Hack to ensure no data is preserved on issue before starting the spec,
+        # to avoid false negatives
+        reloaded_issue = Issue.find(issue.id)
+
+        pipeline_routes = lambda do |merge_requests|
+          merge_requests.map { |mr| mr.head_pipeline&.project&.full_path }
+        end
+
+        closing_mr_other_project.update!(head_pipeline: create(:ci_pipeline))
+        control_count = ActiveRecord::QueryRecorder.new { service.execute(reloaded_issue).each(&pipeline_routes) }
+
+        closing_mr.update!(head_pipeline: create(:ci_pipeline))
+
+        expect { service.execute(issue).each(&pipeline_routes) }
+          .not_to exceed_query_limit(control_count)
+      end
     end
   end