class Rails::Generators::Db::System::ChangeGenerator

def initialize(*)
  super
  unless DATABASES.include?(options[:to])
    raise Error, "Invalid value for --to option. Supported preconfigurations are: #{DATABASES.join(", ")}."
  end
  opt = options.dup
  opt[:database] ||= opt[:to]
  self.options = opt.freeze
end