Eliminate N+1 queries in /api/groups/:id

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15475/diffs, a
significant amount of work went into eliminating N+1 queries in the
/api/groups/:id/projects endpoint. We can reuse the
`Entities::Project.prepare_relation` call on the projects.

In a group with 2,573 projects on GitLab.com, this change significantly
improves performance:

* 18019 SQL queries down to 21
* Time spent in DB: 70 s down to 384 ms

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/49845

changelogs/unreleased/sh-preload-associations-for-group-api.yml

+---
+title: Eliminate N+1 queries in /api/groups/:id
+merge_request: 24513
+author:
+type: performance

lib/api/entities.rb

@@ -344,19 +344,23 @@ module API
 
     class GroupDetail < Group
       expose :projects, using: Entities::Project do |group, options|
-        GroupProjectsFinder.new(
+        projects = GroupProjectsFinder.new(
           group: group,
           current_user: options[:current_user],
           options: { only_owned: true }
         ).execute
+
+        Entities::Project.prepare_relation(projects)
       end
 
       expose :shared_projects, using: Entities::Project do |group, options|
-        GroupProjectsFinder.new(
+        projects = GroupProjectsFinder.new(
           group: group,
           current_user: options[:current_user],
           options: { only_shared: true }
         ).execute
+
+        Entities::Project.prepare_relation(projects)
       end
     end
 

spec/requests/api/groups_spec.rb

@@ -382,6 +382,20 @@ describe API::Groups do
         expect(response_project_ids(json_response, 'shared_projects'))
           .to contain_exactly(projects[:public].id, projects[:internal].id)
       end
+
+      it 'avoids N+1 queries' do
+        get api("/groups/#{group1.id}", admin)
+
+        control_count = ActiveRecord::QueryRecorder.new do
+          get api("/groups/#{group1.id}", admin)
+        end.count
+
+        create(:project, namespace: group1)
+
+        expect do
+          get api("/groups/#{group1.id}", admin)
+        end.not_to exceed_query_limit(control_count)
+      end
     end
 
     context "when authenticated as admin" do