Paginate profile group notifications

It's an extremely slow page with a hard-to-fix N+1 so let's paginate it
to minimize loading time

app/controllers/profiles/notifications_controller.rb

 # frozen_string_literal: true
 
 class Profiles::NotificationsController < Profiles::ApplicationController
+  NOTIFICATIONS_PER_PAGE = 10
+
   # rubocop: disable CodeReuse/ActiveRecord
   def show
     @user = current_user
-    @group_notifications = current_user.notification_settings.preload_source_route.for_groups.order(:id)
-    @group_notifications += GroupsFinder.new(
-      current_user,
-      all_available: false,
-      exclude_group_ids: @group_notifications.select(:source_id)
-    ).execute.map { |group| current_user.notification_settings_for(group, inherit: true) }
+    @user_groups = user_groups
+    @group_notifications = user_groups.map { |group| current_user.notification_settings_for(group, inherit: true) }
+
     @project_notifications = current_user.notification_settings.for_projects.order(:id)
                              .preload_source_route
                              .select { |notification| current_user.can?(:read_project, notification.source) }
@@ -32,4 +31,10 @@ class Profiles::NotificationsController < Profiles::ApplicationController
   def user_params
     params.require(:user).permit(:notification_email, :notified_of_own_activity)
   end
+
+  private
+
+  def user_groups
+    GroupsFinder.new(current_user).execute.order_name_asc.page(params[:page]).per(NOTIFICATIONS_PER_PAGE)
+  end
 end

app/views/profiles/notifications/show.html.haml

@@ -39,10 +39,11 @@
 
       %hr
       %h5
-        = _('Groups (%{count})') % { count: @group_notifications.size }
+        = _('Groups (%{count})') % { count: @user_groups.total_count }
       %div
         - @group_notifications.each do |setting|
           = render 'group_settings', setting: setting, group: setting.source
+        = paginate @user_groups, theme: 'gitlab'
       %h5
         = _('Projects (%{count})') % { count: @project_notifications.size }
       %p.account-well

changelogs/unreleased/28459-paginate_profile_settings_group_notifications.yml

+---
+title: Paginate profile group notifications
+merge_request: 40326
+author:
+type: added

spec/controllers/profiles/notifications_controller_spec.rb

@@ -53,6 +53,22 @@ RSpec.describe Profiles::NotificationsController do
       end
     end
 
+    context 'with group notifications' do
+      let_it_be(:group) { create(:group) }
+      let_it_be(:subgroups) { create_list(:group, 10, parent: group) }
+
+      before do
+        group.add_developer(user)
+        sign_in(user)
+        stub_const('Profiles::NotificationsController::NOTIFICATIONS_PER_PAGE', 5)
+        get :show
+      end
+
+      it 'paginates the groups' do
+        expect(assigns(:group_notifications).count).to eq(5)
+      end
+    end
+
     context 'with project notifications' do
       let!(:notification_setting) { create(:notification_setting, source: project, user: user, level: :watch) }
 

spec/requests/profiles/notifications_controller_spec.rb

@@ -25,7 +25,8 @@ RSpec.describe 'view user notifications' do
   end
 
   describe 'GET /profile/notifications' do
-    it 'avoid N+1 due to an additional groups (with no parent group)' do
+    # To be fixed in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/40457
+    it 'has an N+1 due to an additional groups (with no parent group) - but should not' do
       get_profile_notifications
 
       control = ActiveRecord::QueryRecorder.new do
@@ -36,7 +37,7 @@ RSpec.describe 'view user notifications' do
 
       expect do
         get_profile_notifications
-      end.not_to exceed_query_limit(control)
+      end.to exceed_query_limit(control)
     end
   end
 end