    # File lib/action_view/helpers/form_helper.rb, line 145
          def fields_for(object_name, *args, &block)
            raise ArgumentError, "Missing block" unless block_given?
            options = args.last.is_a?(Hash) ? args.pop : {}
            object  = args.first

            builder = options[:builder] || ActionView::Base.default_form_builder
            yield builder.new(object_name, object, self, options, block)
          end