correctly presence check of `application_record.rb` in plugin

activerecord/lib/rails/generators/active_record/model/model_generator.rb

@@ -45,7 +45,13 @@ def parent_class_name
         def determine_default_parent_class
           application_record = nil
 
-          in_root { application_record = File.exist?('app/models/application_record.rb') }
+          in_root do
+            application_record = if mountable_engine?
+              File.exist?("app/models/#{namespaced_path}/application_record.rb")
+            else
+              File.exist?('app/models/application_record.rb')
+            end
+          end
 
           if application_record
             "ApplicationRecord"

railties/test/generators/plugin_generator_test.rb

@@ -621,6 +621,15 @@ def test_scaffold_generator_for_mountable_api_plugins
     assert_no_directory "app/views"
   end
 
+  def test_model_with_existent_application_record_in_mountable_engine
+    run_generator [destination_root, '--mountable']
+    capture(:stdout) do
+      `#{destination_root}/bin/rails g model article`
+    end
+
+    assert_file "app/models/bukkits/article.rb", /class Article < ApplicationRecord/
+  end
+
 protected
   def action(*args, &block)
     silence(:stdout){ generator.send(*args, &block) }