# File lib/action_view/helpers/form_helper.rb, line 605
      def fields_for(record_name, record_object = nil, options = {}, &block)
        builder = instantiate_builder(record_name, record_object, options, &block)
        output = capture(builder, &block)
        output.concat builder.hidden_field(:id) if output && options[:hidden_field_id] && !builder.emitted_hidden_id?
        output
      end