Preload labels in EpicsController#index

Preload labels in EpicsController#index to avoid N+1 queries.

ee/app/controllers/groups/epics_controller.rb

@@ -99,7 +99,7 @@ class Groups::EpicsController < Groups::ApplicationController
   end
 
   def preload_for_collection
-    @preload_for_collection ||= [:group, :author]
+    @preload_for_collection ||= [:group, :author, :labels]
   end
 
   def authorize_create_epic!

ee/changelogs/unreleased/ee-11178-fix-n1-queries-in-epics-controller-index.yml

+---
+title: Performance improvement when loading epics list
+merge_request: 13904
+author:
+type: performance

ee/spec/controllers/groups/epics_controller_spec.rb

@@ -116,6 +116,26 @@ describe Groups::EpicsController do
         end
       end
 
+      context "when epic has multiple labels" do
+        render_views
+
+        let(:label) { create(:label) }
+        let!(:labeled_epic) { create(:labeled_epic, group: group, labels: [label]) }
+
+        it 'does not cause N+1 queries' do
+          get :index, params: { group_id: group }
+
+          control_count = ActiveRecord::QueryRecorder.new do
+            get :index, params: { group_id: group }
+          end
+
+          label = create(:label)
+          create(:labeled_epic, group: group, labels: [label])
+
+          expect { get :index, params: { group_id: group } }.not_to exceed_query_limit(control_count)
+        end
+      end
+
       context 'when format is JSON' do
         before do
           allow(Kaminari.config).to receive(:default_per_page).and_return(1)