Avoid N+1 queries in epic reference

* We can avoid N+1 in epic autocomplete by preloading epic groups.
* We can also optimize epic cross reference by using just group id instead
of using epic's group (and group's projects).

ee/app/models/ee/epic.rb

@@ -387,13 +387,22 @@ module EE
     def to_reference(from = nil, full: false)
       reference = "#{self.class.reference_prefix}#{iid}"
 
-      return reference unless (cross_referenced?(from) && !group.projects.include?(from)) || full
+      return reference unless full || cross_referenced?(from)
 
       "#{group.full_path}#{reference}"
     end
 
     def cross_referenced?(from)
-      from && from != group
+      return false unless from
+
+      case from
+      when ::Group
+        from.id != group_id
+      when ::Project
+        from.namespace_id != group_id
+      else
+        true
+      end
     end
 
     def ancestors

ee/app/services/ee/groups/autocomplete_service.rb

@@ -2,6 +2,7 @@
 module EE
   module Groups
     module AutocompleteService
+      # rubocop: disable CodeReuse/ActiveRecord
       def epics(confidential_only: false)
         finder_params = { group_id: group.id }
         finder_params[:confidential] = true if confidential_only.present?
@@ -10,8 +11,10 @@ module EE
         # See https://gitlab.com/gitlab-org/gitlab/issues/6837
         EpicsFinder.new(current_user, finder_params)
           .execute
+          .preload(:group)
           .select(:iid, :title, :group_id)
       end
+      # rubocop: enable CodeReuse/ActiveRecord
 
       def vulnerabilities
         ::Autocomplete::VulnerabilitiesAutocompleteFinder

ee/changelogs/unreleased/n1_cross_ref.yml

+---
+title: Minimize number of SQL queries on epic autocompletion.
+merge_request: 61885
+author:
+type: performance

ee/spec/models/epic_spec.rb

@@ -568,6 +568,14 @@ RSpec.describe Epic do
         expect(epic.to_reference(group_project, full: true)).to eq('group-a&1')
       end
     end
+
+    it 'avoids additional SQL queries' do
+      epic # pre-create the epic
+
+      recorder = ActiveRecord::QueryRecorder.new { epic.to_reference(project) }
+
+      expect(recorder.count).to be_zero
+    end
   end
 
   describe '#has_children?' do

ee/spec/controllers/ee/groups/autocomplete_sources_controller_spec.rb → ee/spec/requests/ee/groups/autocomplete_sources_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Groups::AutocompleteSourcesController do
+RSpec.describe 'groups autocomplete' do
   let_it_be(:user) { create(:user) }
   let_it_be_with_reload(:group) { create(:group, :private) }
   let_it_be(:epic) { create(:epic, group: group) }
@@ -32,9 +32,8 @@ RSpec.describe Groups::AutocompleteSourcesController do
     with_them do
       it 'returns the correct response', :aggregate_failures do
         issues = Array(expected).flat_map { |sym| public_send(sym) }
-        params = { group_id: group, issue_types: issue_types }.compact
 
-        get :issues, params: params
+        get issues_group_autocomplete_sources_path(group, issue_types: issue_types)
 
         expect(response).to have_gitlab_http_status(:ok)
         expect(json_response).to be_an(Array)
@@ -47,19 +46,29 @@ RSpec.describe Groups::AutocompleteSourcesController do
 
   describe '#epics' do
     it 'returns 200 status' do
-      get :epics, params: { group_id: group }
+      get epics_group_autocomplete_sources_path(group)
 
       expect(response).to have_gitlab_http_status(:ok)
     end
 
     it 'returns the correct response' do
-      get :epics, params: { group_id: group }
+      get epics_group_autocomplete_sources_path(group)
 
       expect(json_response).to be_an(Array)
       expect(json_response.first).to include(
         'iid' => epic.iid, 'title' => epic.title, 'reference' => epic.to_reference(epic.group)
       )
     end
+
+    it 'avoids N+1 queries' do
+      control = ActiveRecord::QueryRecorder.new(skip_cached: false) { get epics_group_autocomplete_sources_path(group) }
+
+      create_list(:epic, 3, group: group)
+
+      expect do
+        get epics_group_autocomplete_sources_path(group)
+      end.not_to exceed_all_query_limit(control)
+    end
   end
 
   describe '#vulnerabilities' do
@@ -72,13 +81,13 @@ RSpec.describe Groups::AutocompleteSourcesController do
     end
 
     it 'returns 200 status' do
-      get :vulnerabilities, params: { group_id: group }
+      get vulnerabilities_group_autocomplete_sources_path(group)
 
       expect(response).to have_gitlab_http_status(:ok)
     end
 
     it 'returns the correct response', :aggregate_failures do
-      get :vulnerabilities, params: { group_id: group }
+      get vulnerabilities_group_autocomplete_sources_path(group)
 
       expect(json_response).to be_an(Array)
       expect(json_response.first).to include(
@@ -89,13 +98,13 @@ RSpec.describe Groups::AutocompleteSourcesController do
 
   describe '#commands' do
     it 'returns 200 status' do
-      get :commands, params: { group_id: group, type: 'Epic', type_id: epic.iid }
+      get commands_group_autocomplete_sources_path(group, type: 'Epic', type_id: epic.iid)
 
       expect(response).to have_gitlab_http_status(:ok)
     end
 
     it 'returns the correct response' do
-      get :commands, params: { group_id: group, type: 'Epic', type_id: epic.iid }
+      get commands_group_autocomplete_sources_path(group, type: 'Epic', type_id: epic.iid)
 
       expect(json_response).to be_an(Array)
       expect(json_response).to include(
@@ -107,7 +116,7 @@ RSpec.describe Groups::AutocompleteSourcesController do
     end
 
     it 'handles new epics' do
-      get :commands, params: { group_id: group, type: 'Epic', type_id: nil }
+      get commands_group_autocomplete_sources_path(group, type: 'Epic', type_id: nil)
 
       expect(json_response).to be_an(Array)
       expect(json_response).to include(

spec/controllers/groups/autocomplete_sources_controller_spec.rb → spec/requests/groups/autocomplete_sources_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe Groups::AutocompleteSourcesController do
+RSpec.describe 'groups autocomplete' do
   let_it_be(:user) { create(:user) }
   let_it_be_with_reload(:group) { create(:group, :private) }
 
@@ -35,9 +35,8 @@ RSpec.describe Groups::AutocompleteSourcesController do
     with_them do
       it 'returns the correct response', :aggregate_failures do
         issues = Array(expected).flat_map { |sym| public_send(sym) }
-        params = { group_id: group, issue_types: issue_types }.compact
 
-        get :issues, params: params
+        get issues_group_autocomplete_sources_path(group, issue_types: issue_types)
 
         expect(response).to have_gitlab_http_status(:ok)
         expect(json_response).to be_an(Array)
@@ -57,7 +56,7 @@ RSpec.describe Groups::AutocompleteSourcesController do
       create(:milestone, group: sub_group)
       group_milestone = create(:milestone, group: group)
 
-      get :milestones, params: { group_id: group }
+      get milestones_group_autocomplete_sources_path(group)
 
       expect(response).to have_gitlab_http_status(:ok)
       expect(json_response.count).to eq(1)