Optimize API /groups/:id/projects by preloading associations

Closes #40308

changelogs/unreleased/sh-optimize-groups-api.yml

+---
+title: Optimize API /groups/:id/projects by preloading fork_networks table
+merge_request:
+author:
+type: performance

lib/api/groups.rb

@@ -172,6 +172,7 @@ module API
       get ":id/projects" do
         group = find_group!(params[:id])
         projects = GroupProjectsFinder.new(group: group, current_user: current_user, params: project_finder_params).execute
+        projects = projects.preload(:fork_network, :forked_project_link, :project_feature, :project_group_links, :tags, :taggings, :group, :namespace)
         projects = reorder_projects(projects)
         entity = params[:simple] ? Entities::BasicProjectDetails : Entities::Project
         present paginate(projects), with: entity, current_user: current_user

spec/requests/api/groups_spec.rb

@@ -401,6 +401,20 @@ describe API::Groups do
 
         expect(response).to have_gitlab_http_status(404)
       end
+
+      it 'avoids N+1 queries' do
+        get api("/groups/#{group1.id}/projects", admin)
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          get api("/groups/#{group1.id}/projects", admin)
+        end.count
+
+        create(:project, namespace: group1)
+
+        expect do
+          get api("/groups/#{group1.id}/projects", admin)
+        end.not_to exceed_query_limit(control_count)
+      end
     end
 
     context 'when using group path in URL' do