Fix N+1 in projects REST endpoint and forked projects

When we retrieve projects from the REST endpoint and there are
several forks inside, there are several N+1.

In this commit we fix several of those.

changelogs/unreleased/21121-fj-fix-n-1-projects-endpoint-forked-projects.yml

+---
+title: Fix N+1 in projects REST endpoint with forked projects
+merge_request: 57798
+author:
+type: performance

lib/api/entities/basic_project_details.rb

@@ -8,11 +8,10 @@ module API
       expose :default_branch, if: -> (project, options) { Ability.allowed?(options[:current_user], :download_code, project) }
       # Avoids an N+1 query: https://github.com/mbleigh/acts-as-taggable-on/issues/91#issuecomment-168273770
       expose :tag_list do |project|
-        # project.tags.order(:name).pluck(:name) is the most suitable option
-        # to avoid loading all the ActiveRecord objects but, if we use it here
-        # it override the preloaded associations and makes a query
-        # (fixed in https://github.com/rails/rails/pull/25976).
-        project.tags.map(&:name).sort
+        # Tags is a preloaded association. If we perform then sorting
+        # through the database, it will trigger a new query, ending up
+        # in an N+1 if we have several projects
+        project.tags.pluck(:name).sort # rubocop:disable CodeReuse/ActiveRecord
       end
 
       expose :ssh_url_to_repo, :http_url_to_repo, :web_url, :readme_url

lib/api/entities/project.rb

@@ -135,7 +135,7 @@ module API
                                 .preload(project_group_links: { group: :route },
                                          fork_network: :root_project,
                                          fork_network_member: :forked_from_project,
-                                         forked_from_project: [:route, :forks, :tags, namespace: :route])
+                                         forked_from_project: [:route, :forks, :tags, :group, :project_feature, namespace: [:route, :owner]])
       end
       # rubocop: enable CodeReuse/ActiveRecord
 

spec/requests/api/projects_spec.rb

@@ -810,6 +810,31 @@ RSpec.describe API::Projects do
         end
       end
     end
+
+    context 'with forked projects', :use_clean_rails_memory_store_caching do
+      include ProjectForksHelper
+
+      let_it_be(:admin) { create(:admin) }
+
+      it 'avoids N+1 queries' do
+        get api('/projects', admin)
+
+        base_project = create(:project, :public, namespace: admin.namespace)
+
+        fork_project1 = fork_project(base_project, admin, namespace: create(:user).namespace)
+        fork_project2 = fork_project(fork_project1, admin, namespace: create(:user).namespace)
+
+        control = ActiveRecord::QueryRecorder.new do
+          get api('/projects', admin)
+        end
+
+        fork_project(fork_project2, admin, namespace: create(:user).namespace)
+
+        expect do
+          get api('/projects', admin)
+        end.not_to exceed_query_limit(control.count)
+      end
+    end
   end
 
   describe 'POST /projects' do