Fix N+1 for group container repositories view

The two offennding things were pre-loading the namespace, and also
loading resources for destroy_path. We don't need destroy_path for group
view as it's read-only so skip that

app/controllers/groups/registry/repositories_controller.rb

@@ -16,7 +16,7 @@ module Groups
 
             render json: ContainerRepositoriesSerializer
               .new(current_user: current_user)
-              .represent(@images)
+              .represent_read_only(@images)
           end
         end
       end

app/models/container_repository.rb

@@ -11,7 +11,7 @@ class ContainerRepository < ApplicationRecord
   delegate :client, to: :registry
 
   scope :ordered, -> { order(:name) }
-  scope :with_api_entity_associations, -> { preload(:project) }
+  scope :with_api_entity_associations, -> { preload(project: [:route, { namespace: :route }]) }
 
   # rubocop: disable CodeReuse/ServiceClass
   def registry

app/serializers/container_repositories_serializer.rb

@@ -2,4 +2,8 @@
 
 class ContainerRepositoriesSerializer < BaseSerializer
   entity ContainerRepositoryEntity
+
+  def represent_read_only(resource)
+    represent(resource, except: [:destroy_path])
+  end
 end

changelogs/unreleased/fix_group_container_repositories_n_1.yml

+---
+title: Fix N+1 for group container repositories view
+merge_request: 18979
+author:
+type: performance

spec/requests/groups/registry/repositories_controller_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe Groups::Registry::RepositoriesController do
+  let_it_be(:group, reload: true) { create(:group) }
+  let_it_be(:user) { create(:user) }
+
+  before do
+    stub_container_registry_config(enabled: true)
+
+    group.add_reporter(user)
+    login_as(user)
+  end
+
+  describe 'GET groups/:group_id/-/container_registries.json' do
+    it 'avoids N+1 queries' do
+      project = create(:project, group: group)
+      create(:container_repository, project: project)
+      endpoint = group_container_registries_path(group, format: :json)
+
+      control_count = ActiveRecord::QueryRecorder.new(skip_cached: false) { get(endpoint) }.count
+
+      create_list(:project, 2, group: group).each do |project|
+        create_list(:container_repository, 2, project: project)
+      end
+
+      expect { get(endpoint) }.not_to exceed_all_query_limit(control_count)
+
+      # sanity check that response is 200
+      expect(response).to have_http_status(200)
+      repositories = json_response
+      expect(repositories.count).to eq(5)
+    end
+  end
+end