Prevent Group API N+1 loading group plans

When querying the Groups Project API and including subgroups,
N+1 queries can take place when loading plans for all the groups
in the hierarchy. Plans are only applied at the top-level group
so it is unnecessarily inefficient to load plans for each project
and associated group in the hierarchy. This change loads the plans
once and memoizes them for all project groups.

Changelog: fixed
EE: true

app/models/preloaders/single_hierarchy_project_group_plans_preloader.rb

+# frozen_string_literal: true
+
+module Preloaders
+  class SingleHierarchyProjectGroupPlansPreloader
+    attr_reader :projects
+
+    def initialize(projects_relation)
+      @projects = projects_relation
+    end
+
+    def execute
+      # no-op in FOSS
+    end
+  end
+end
+
+Preloaders::SingleHierarchyProjectGroupPlansPreloader.prepend_mod_with('Preloaders::SingleHierarchyProjectGroupPlansPreloader')

config/feature_flags/development/group_project_api_preload_plans.yml

+---
+name: group_project_api_preload_plans
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77538
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/350176
+milestone: '14.7'
+type: development
+group: group::authentication and authorization
+default_enabled: false

ee/app/models/ee/preloaders/single_hierarchy_project_group_plans_preloader.rb

+# frozen_string_literal: true
+
+module EE
+  module Preloaders
+    module SingleHierarchyProjectGroupPlansPreloader
+      def execute
+        return unless ::Feature.enabled?(:group_project_api_preload_plans, default_enabled: :yaml)
+        return unless ::Gitlab::CurrentSettings.should_check_namespace_plan?
+        return unless project = projects.take
+
+        plans = project.namespace.root_ancestor.plans
+
+        preload_plans(plans)
+      end
+
+      def preload_plans(plans)
+        return unless plans.any?
+
+        projects.each do |project|
+          project.namespace.memoized_plans = plans
+        end
+      end
+    end
+  end
+end

ee/spec/requests/api/groups_spec.rb

@@ -632,6 +632,54 @@ RSpec.describe API::Groups do
         end
       end
     end
+
+    context 'when namespace license checks are enabled', :saas do
+      before do
+        enable_namespace_license_check!
+      end
+
+      context 'when there are plans and projects' do
+        let(:group) { create(:group_with_plan, plan: :ultimate_plan) }
+
+        before do
+          subgroup = create(:group, parent: group)
+          create(:project, group: group)
+          create(:project, group: subgroup)
+        end
+
+        it 'only loads plans once' do
+          expect(Plan).to receive(:hosted_plans_for_namespaces).once.and_call_original
+
+          get api("/groups/#{group.id}/projects", user), params: { include_subgroups: true }
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+
+        context 'when the feature flag is disabled' do
+          before do
+            stub_feature_flags(group_project_api_preload_plans: false)
+          end
+
+          it 'does not preload plans' do
+            expect(Plan).to receive(:hosted_plans_for_namespaces).at_least(:twice).and_call_original
+
+            get api("/groups/#{group.id}/projects", user), params: { include_subgroups: true }
+
+            expect(response).to have_gitlab_http_status(:ok)
+          end
+        end
+      end
+
+      context 'when there are no projects' do
+        let(:group) { create(:group) }
+
+        it 'completes the request without error' do
+          get api("/groups/#{group.id}/projects", user), params: { include_subgroups: true }
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
+    end
   end
 
   describe 'GET group/:id/audit_events' do

lib/api/groups.rb

@@ -84,10 +84,11 @@ module API
         paginate(projects)
       end
 
-      def present_projects(params, projects)
+      def present_projects(params, projects, single_hierarchy: false)
         options = {
           with: params[:simple] ? Entities::BasicProjectDetails : Entities::Project,
-          current_user: current_user
+          current_user: current_user,
+          single_hierarchy: single_hierarchy
         }
 
         projects, options = with_custom_attributes(projects, options)
@@ -306,7 +307,7 @@ module API
 
         projects = find_group_projects(params, finder_options)
 
-        present_projects(params, projects)
+        present_projects(params, projects, single_hierarchy: true)
       end
 
       desc 'Get a list of shared projects in this group' do

lib/api/projects_relation_builder.rb

@@ -13,6 +13,7 @@ module API
         preload_repository_cache(projects_relation)
 
         Preloaders::UserMaxAccessLevelInProjectsPreloader.new(projects_relation, options[:current_user]).execute if options[:current_user]
+        Preloaders::SingleHierarchyProjectGroupPlansPreloader.new(projects_relation).execute if options[:single_hierarchy]
 
         projects_relation
       end