Fix N+1 for project/:id/issues API endpoint

Preload milestones and closed_by users

app/models/issue.rb

@@ -87,11 +87,15 @@ class Issue < ApplicationRecord
   scope :order_created_at_desc, -> { reorder(created_at: :desc) }
 
   scope :preload_associated_models, -> { preload(:assignees, :labels, project: :namespace) }
-  scope :with_api_entity_associations, -> { preload(:timelogs, :assignees, :author, :notes, :labels, project: [:route, { namespace: :route }] ) }
   scope :with_label_attributes, ->(label_attributes) { joins(:labels).where(labels: label_attributes) }
   scope :with_alert_management_alerts, -> { joins(:alert_management_alert) }
   scope :with_prometheus_alert_events, -> { joins(:issues_prometheus_alert_events) }
   scope :with_self_managed_prometheus_alert_events, -> { joins(:issues_self_managed_prometheus_alert_events) }
+  scope :with_api_entity_associations, -> {
+    preload(:timelogs, :closed_by, :assignees, :author, :notes, :labels,
+      milestone: { project: [:route, { namespace: :route }] },
+      project: [:route, { namespace: :route }])
+  }
 
   scope :public_only, -> { where(confidential: false) }
   scope :confidential_only, -> { where(confidential: true) }

changelogs/unreleased/id-fix-n1-for-issues-api.yml

+---
+title: Fix N+1 for project/:id/issues API endpoint
+merge_request: 37508
+author:
+type: performance

spec/requests/api/issues/get_project_issues_spec.rb

@@ -180,12 +180,15 @@ RSpec.describe API::Issues do
     it 'avoids N+1 queries' do
       get api("/projects/#{project.id}/issues", user)
 
-      create_list(:issue, 3, project: project)
+      create_list(:issue, 3, project: project, closed_by: user)
 
       control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) do
         get api("/projects/#{project.id}/issues", user)
       end.count
 
+      milestone = create(:milestone, project: project)
+      create(:issue, project: project, milestone: milestone, closed_by: create(:user))
+
       expect do
         get api("/projects/#{project.id}/issues", user)
       end.not_to exceed_all_query_limit(control_count)