Preload group root ancestor for Group Projects API

The group projects API relies on the root ancestor namespace when
determining many settings and features. Preload this association
to prevent N+1s.

Changelog: fixed

config/feature_flags/development/group_projects_api_preload_groups.yml

+---
+name: group_projects_api_preload_groups
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81838
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/354372
+milestone: '14.9'
+type: development
+group: group::authentication and authorization
+default_enabled: false

lib/api/projects_relation_builder.rb

@@ -14,6 +14,7 @@ module API
 
         Preloaders::UserMaxAccessLevelInProjectsPreloader.new(projects_relation, options[:current_user]).execute if options[:current_user]
         Preloaders::SingleHierarchyProjectGroupPlansPreloader.new(projects_relation).execute if options[:single_hierarchy]
+        preload_groups(projects_relation) if options[:with] == Entities::Project
 
         projects_relation
       end
@@ -40,6 +41,25 @@ module API
       def repositories_for_preload(projects_relation)
         projects_relation.map(&:repository)
       end
+
+      # For all projects except those in a user namespace, the `namespace`
+      # and `group` are identical. Preload the group when it's not a user namespace.
+      def preload_groups(projects_relation)
+        return unless Feature.enabled?(:group_projects_api_preload_groups)
+
+        group_projects = projects_for_group_preload(projects_relation)
+        groups = group_projects.map(&:namespace)
+
+        Preloaders::GroupRootAncestorPreloader.new(groups).execute
+
+        group_projects.each do |project|
+          project.group = project.namespace
+        end
+      end
+
+      def projects_for_group_preload(projects_relation)
+        projects_relation.select { |project| project.namespace.type == Group.sti_name }
+      end
     end
   end
 end

spec/requests/api/groups_spec.rb

@@ -1164,17 +1164,47 @@ RSpec.describe API::Groups do
       end
 
       context 'when include_subgroups is true' do
-        it "returns projects including those in subgroups" do
+        before do
           subgroup = create(:group, parent: group1)
+          subgroup2 = create(:group, parent: subgroup)
+
           create(:project, group: subgroup)
           create(:project, group: subgroup)
+          create(:project, group: subgroup2)
+
+          group1.reload
+        end
+
+        it "only looks up root ancestor once and returns projects including those in subgroups" do
+          expect(Namespace).to receive(:find_by).with(id: group1.id.to_s).once.and_call_original # For the group sent in the API call
+          expect(Namespace).to receive(:find_by).with(id: group1.traversal_ids.first).once.and_call_original # root_ancestor direct lookup
+          expect(Namespace).to receive(:joins).with(start_with('INNER JOIN (SELECT id, traversal_ids[1]')).once.and_call_original # All-in-one root_ancestor query
 
           get api("/groups/#{group1.id}/projects", user1), params: { include_subgroups: true }
 
           expect(response).to have_gitlab_http_status(:ok)
           expect(response).to include_pagination_headers
           expect(json_response).to be_an(Array)
-          expect(json_response.length).to eq(5)
+          expect(json_response.length).to eq(6)
+        end
+
+        context 'when group_projects_api_preload_groups feature is disabled' do
+          before do
+            stub_feature_flags(group_projects_api_preload_groups: false)
+          end
+
+          it 'looks up the root ancestor multiple times' do
+            expect(Namespace).to receive(:find_by).with(id: group1.id.to_s).once.and_call_original
+            expect(Namespace).to receive(:find_by).with(id: group1.traversal_ids.first).at_least(:twice).and_call_original
+            expect(Namespace).not_to receive(:joins).with(start_with('INNER JOIN (SELECT id, traversal_ids[1]'))
+
+            get api("/groups/#{group1.id}/projects", user1), params: { include_subgroups: true }
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(response).to include_pagination_headers
+            expect(json_response).to be_an(Array)
+            expect(json_response.length).to eq(6)
+          end
         end
       end