Overriding Rails' Default database.yml Generation in Application Templates

You can certainly change what ends up in your application's database configuration, but there isn’t an officially supported hook to “override” the AppBuilder method that creates database.yml. The Rails application template API is designed to let you modify the generated application (for example, by copying your own database.yml file) rather than replacing internal generator behavior outright.

If you’re happy with the approach of copying your own version after the app is generated using:

RUBYCopied
def self.source_paths
  [__dir__]
end

template "config/databases/postgresql.yml", "config/database.yml"

that is the recommended way to tailor the generated database configuration.

If you really need to replace the behavior of the default AppBuilder#create_database_yml method, you’d have to monkey-patch AppBuilder in your template. For example, at the top of your template you could reopen the class and override the method:

RUBYCopied
class AppBuilder
  def create_database_yml
    say_status "custom", "Generating custom database.yml", :green
    # Replace with your custom copy_template action.
    template "config/databases/postgresql.yml", "config/database.yml"
  end
end

Keep in mind that monkey-patching internal Rails generator classes is not officially supported. It relies on internal APIs that may change in future Rails releases, so it can make your template less future-proof. The recommended approach is to remove (or overwrite) the generated database.yml after the default generation—just as you’re already doing with the template command.

In short, while you can override the method via monkey-patching, it’s preferable and more stable to simply remove or replace the generated file in your application template.