Fix N + 1 for MilestonesController#merge_requests

Preload merge requests associations to avoid N + 1
on milestones#show page

app/controllers/concerns/milestone_actions.rb

@@ -20,7 +20,7 @@ module MilestoneActions
       format.html { redirect_to milestone_redirect_path }
       format.json do
         render json: tabs_json("shared/milestones/_merge_requests_tab", {
-          merge_requests: @milestone.sorted_merge_requests(current_user), # rubocop:disable Gitlab/ModuleWithInstanceVariables
+          merge_requests: @milestone.sorted_merge_requests(current_user).preload_milestoneish_associations, # rubocop:disable Gitlab/ModuleWithInstanceVariables
           show_project_name: Gitlab::Utils.to_boolean(params[:show_project_name])
         })
       end

app/models/merge_request.rb

@@ -359,6 +359,8 @@ class MergeRequest < ApplicationRecord
   scope :preload_metrics, -> (relation) { preload(metrics: relation) }
   scope :preload_project_and_latest_diff, -> { preload(:source_project, :latest_merge_request_diff) }
   scope :preload_latest_diff_commit, -> { preload(latest_merge_request_diff: :merge_request_diff_commits) }
+  scope :preload_milestoneish_associations, -> { preload_routables.preload(:assignees, :labels) }
+
   scope :with_web_entity_associations, -> { preload(:author, target_project: [:project_feature, group: [:route, :parent], namespace: :route]) }
 
   scope :with_auto_merge_enabled, -> do

changelogs/unreleased/id-preload-associations-to-milestones-mrs.yml

+---
+title: 'Fix N + 1 for MilestonesController#merge_requests'
+merge_request: 57980
+author:
+type: performance

spec/requests/groups/milestones_controller_spec.rb

@@ -4,8 +4,8 @@ require 'spec_helper'
 
 RSpec.describe Groups::MilestonesController do
   context 'N+1 DB queries' do
-    let(:user) { create(:user) }
-    let!(:public_group) { create(:group, :public) }
+    let_it_be(:user) { create(:user) }
+    let_it_be(:public_group) { create(:group, :public) }
 
     let!(:public_project_with_private_issues_and_mrs) do
       create(:project, :public, :issues_private, :merge_requests_private, group: public_group)
@@ -53,5 +53,25 @@ RSpec.describe Groups::MilestonesController do
         expect { get show_path }.not_to exceed_all_query_limit(control)
       end
     end
+
+    describe 'GET #merge_requests' do
+      let(:milestone) { create(:milestone, group: public_group) }
+      let(:project) { create(:project, :public, :merge_requests_enabled, :issues_enabled, group: public_group) }
+      let!(:merge_request) { create(:merge_request, milestone: milestone, source_project: project) }
+
+      def perform_request
+        get merge_requests_group_milestone_path(public_group, milestone, format: :json)
+      end
+
+      it 'avoids N+1 database queries' do
+        perform_request # warm up the cache
+
+        control_count = ActiveRecord::QueryRecorder.new { perform_request }.count
+
+        create(:merge_request, milestone: milestone, source_project: project, source_branch: 'fix')
+
+        expect { perform_request }.not_to exceed_query_limit(control_count)
+      end
+    end
   end
 end