Fix N+1 queries with /api/v4/groups endpoint

This is an EE-only issue since `ldap_group_link` is not in CE.

Fix for gitlab-org/gitlab-ce#42030

changelogs/unreleased-ee/sh-fix-nplus-one-groups-api.yml

+---
+title: FIx N+1 queries with /api/v4/groups endpoint
+merge_request:
+author:
+type: performance

lib/api/groups.rb

@@ -54,6 +54,8 @@ module API
         find_params[:parent] = find_group!(params[:id]) if params[:id]
 
         groups = GroupsFinder.new(current_user, find_params).execute
+        # EE-only
+        groups = groups.preload(:ldap_group_links)
         groups = groups.search(params[:search]) if params[:search].present?
         groups = groups.where.not(id: params[:skip_groups]) if params[:skip_groups].present?
         groups = groups.reorder(params[:order_by] => params[:sort])

spec/requests/api/groups_spec.rb

@@ -31,6 +31,21 @@ describe API::Groups do
         expect(json_response)
           .to satisfy_one { |group| group['name'] == group1.name }
       end
+
+      it 'avoids N+1 queries' do
+        # Establish baseline
+        get api("/groups", admin)
+
+        control = ActiveRecord::QueryRecorder.new do
+          get api("/groups", admin)
+        end
+
+        create(:group)
+
+        expect do
+          get api("/groups", admin)
+        end.not_to exceed_query_limit(control)
+      end
     end
 
     context "when authenticated as user" do