Eliminate N+1 queries in Dashboard::TodosController

This appears to bring down the number of SQL queries on GitLab.com for
my Todos page from 672 to 100.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/43042

app/controllers/dashboard/todos_controller.rb

@@ -10,6 +10,7 @@ class Dashboard::TodosController < Dashboard::ApplicationController
   def index
     @sort = params[:sort]
     @todos = @todos.page(params[:page])
+    @todos = @todos.with_api_entity_associations
     return if redirect_out_of_range(@todos)
   end

app/helpers/todos_helper.rb

@@ -170,7 +170,7 @@ module TodosHelper
   end
   def todo_group_options
-    groups = current_user.authorized_groups.map do |group|
+    groups = current_user.authorized_groups.with_route.map do |group|
       { id: group.id, text: group.full_name }
     end

changelogs/unreleased/sh-optimize-todos-controller.yml

+---
+title: Eliminate N+1 queries in Dashboard::TodosController
+merge_request: 29954
+author:
+type: performance

spec/controllers/dashboard/todos_controller_spec.rb

@@ -44,6 +44,34 @@ describe Dashboard::TodosController do
       end
     end
+    context "with render_views" do
+      render_views
+
+      it 'avoids N+1 queries', :request_store do
+        merge_request = create(:merge_request, source_project: project)
+        create(:todo, project: project, author: author, user: user, target: merge_request)
+        create(:issue, project: project, assignees: [user])
+
+        group = create(:group)
+        group.add_owner(user)
+
+        get :index
+
+        control = ActiveRecord::QueryRecorder.new { get :index }
+
+        create(:issue, project: project, assignees: [user])
+        group_2 = create(:group)
+        group_2.add_owner(user)
+        project_2 = create(:project)
+        project_2.add_developer(user)
+        merge_request_2 = create(:merge_request, source_project: project_2)
+        create(:todo, project: project, author: author, user: user, target: merge_request_2)
+
+        expect { get :index }.not_to exceed_query_limit(control)
+        expect(response.status).to eq(200)
+      end
+    end
+
     context 'group authorization' do
       it 'renders 404 when user does not have read access on given group' do
         unauthorized_group = create(:group, :private)