# File lib/action_view/helpers/form_helper.rb, line 145
      def fields_for(object_name, *args, &proc)
        raise ArgumentError, "Missing block" unless block_given?
        options = args.last.is_a?(Hash) ? args.pop : {}
        object  = args.first
        yield((options[:builder] || FormBuilder).new(object_name, object, self, options, proc))
      end