Fix N+1 queries for Elastic Search milestones scope

app/models/milestone.rb

@@ -28,6 +28,7 @@ class Milestone < ApplicationRecord
 
   scope :order_by_name_asc, -> { order(Arel::Nodes::Ascending.new(arel_table[:title].lower)) }
   scope :reorder_by_due_date_asc, -> { reorder(Gitlab::Database.nulls_last_order('due_date', 'ASC')) }
+  scope :with_api_entity_associations, -> { preload(project: [:project_feature, :route, namespace: :route]) }
 
   validates_associated :milestone_releases, message: -> (_, obj) { obj[:value].map(&:errors).map(&:full_messages).join(",") }
 

ee/changelogs/unreleased/215710-improve-performance-of-search-api-advanced-milestones-scope.yml

+---
+title: Fix N+1 queries for Elastic Search milestones scope.
+merge_request: 33327
+author:
+type: performance

ee/spec/requests/api/search_spec.rb

@@ -212,6 +212,16 @@ RSpec.describe API::Search do
       end
 
       it_behaves_like 'pagination', scope: 'milestones'
+
+      it 'avoids N+1 queries' do
+        control = ActiveRecord::QueryRecorder.new { get api(endpoint, user), params: { scope: 'milestones', search: '*' } }
+        create_list(:milestone, 3, project: project)
+        create_list(:milestone, 2, project: create(:project, :public))
+
+        ensure_elasticsearch_index!
+
+        expect { get api(endpoint, user), params: { scope: 'milestones', search: '*' } }.not_to exceed_query_limit(control.count)
+      end
     end
 
     context 'for users scope', :sidekiq_inline do

lib/api/search.rb

@@ -23,7 +23,8 @@ module API
       SCOPE_PRELOAD_METHOD = {
         merge_requests: :with_api_entity_associations,
         projects: :with_api_entity_associations,
-        issues: :with_api_entity_associations
+        issues: :with_api_entity_associations,
+        milestones: :with_api_entity_associations
       }.freeze
 
       def search(additional_params = {})