correctly check ApplicationRecord is exist in moutable engine

Follow up to 1813b29

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

@@ -41,8 +41,8 @@ def attributes_with_index
 
         # FIXME: Change this file to a symlink once RubyGems 2.5.0 is required.
         def generate_application_record
-          if self.behavior == :invoke && !File.exist?('app/models/application_record.rb')
-            template 'application_record.rb', 'app/models/application_record.rb'
+          if self.behavior == :invoke && !application_record_exist?
+            template 'application_record.rb', application_record_file_name
           end
         end
 
@@ -51,18 +51,22 @@ def parent_class_name
           options[:parent] || determine_default_parent_class
         end
 
-        def determine_default_parent_class
-          application_record = nil
+        def application_record_exist?
+          file_exist = nil
+          in_root { file_exist = File.exist?(application_record_file_name) }
+          file_exist
+        end
 
-          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
+        def application_record_file_name
+          @application_record_file_name ||= if mountable_engine?
+            "app/models/#{namespaced_path}/application_record.rb"
+          else
+            'app/models/application_record.rb'
           end
+        end
 
-          if application_record
+        def determine_default_parent_class
+          if application_record_exist?
             "ApplicationRecord"
           else
             "ActiveRecord::Base"

activerecord/lib/rails/generators/active_record/model/templates/application_record.rb

@@ -1,3 +1,5 @@
+<% module_namespacing do -%>
 class ApplicationRecord < ActiveRecord::Base
   self.abstract_class = true
 end
+<% end -%>

railties/test/generators/plugin_generator_test.rb

@@ -642,6 +642,20 @@ def test_model_with_existent_application_record_in_mountable_engine
     assert_file "app/models/bukkits/article.rb", /class Article < ApplicationRecord/
   end
 
+  def test_generate_application_record_when_does_not_exist_in_mountable_engine
+    run_generator [destination_root, '--mountable']
+    FileUtils.rm "#{destination_root}/app/models/bukkits/application_record.rb"
+    capture(:stdout) do
+      `#{destination_root}/bin/rails g model article`
+    end
+
+    assert_file "#{destination_root}/app/models/bukkits/application_record.rb" do |record|
+      assert_match(/module Bukkits/, record)
+      assert_match(/class ApplicationRecord < ActiveRecord::Base/, record)
+      assert_match(/self.abstract_class = true/, record)
+    end
+  end
+
   def test_after_bundle_callback
     path = 'http://example.org/rails_template'
     template = %{ after_bundle { run 'echo ran after_bundle' } }