Eliminate most N+1 queries loading UserController#calendar_activities

We can reduce a significant number of queries by preloading the
associations for events.

On GitLab.com, for a date that had 456 events, this brought the load
time down from 8.7 to 1.2 s.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58392

changelogs/unreleased/sh-optimize-calendar-activities.yml

+---
+title: Eliminate most N+1 queries loading UserController#calendar_activities
+merge_request: 25697
+author:
+type: performance

lib/gitlab/contributions_calendar.rb

@@ -49,6 +49,7 @@ module Gitlab
       Event.contributions.where(author_id: contributor.id)
         .where(created_at: date.beginning_of_day..date.end_of_day)
         .where(project_id: projects)
+        .with_associations
     end
     # rubocop: enable CodeReuse/ActiveRecord
 

spec/controllers/users_controller_spec.rb

@@ -185,13 +185,32 @@ describe UsersController do
 
     context 'for user' do
       context 'with public profile' do
-        it 'renders calendar_activities' do
-          push_data = Gitlab::DataBuilder::Push.build_sample(project, public_user)
-          EventCreateService.new.push(project, public_user, push_data)
+        let(:issue) { create(:issue, project: project, author: user) }
+        let(:note) { create(:note, noteable: issue, author: user, project: project) }
 
+        render_views
+
+        before do
+          create_push_event
+          create_note_event
+        end
+
+        it 'renders calendar_activities' do
           get :calendar_activities, params: { username: public_user.username }
+
           expect(assigns[:events]).not_to be_empty
         end
+
+        it 'avoids N+1 queries', :request_store do
+          get :calendar_activities, params: { username: public_user.username }
+
+          control = ActiveRecord::QueryRecorder.new { get :calendar_activities, params: { username: public_user.username } }
+
+          create_push_event
+          create_note_event
+
+          expect { get :calendar_activities, params: { username: public_user.username } }.not_to exceed_query_limit(control)
+        end
       end
 
       context 'with private profile' do
@@ -203,6 +222,15 @@ describe UsersController do
           expect(response).to have_gitlab_http_status(:not_found)
         end
       end
+
+      def create_push_event
+        push_data = Gitlab::DataBuilder::Push.build_sample(project, public_user)
+        EventCreateService.new.push(project, public_user, push_data)
+      end
+
+      def create_note_event
+        EventCreateService.new.leave_note(note, public_user)
+      end
     end
   end