instance method fields_for

# File actionpack/lib/action_view/helpers/form_helper.rb, line 1143
      def fields_for(record_or_name_or_array, *args, &block)
        if options.has_key?(:index)
          index = "[#{options[:index]}]"
        elsif defined?(@auto_index)
          self.object_name = @object_name.to_s.sub(/\[\]$/,"")
          index = "[#{@auto_index}]"
        else
          index = ""
        end

        if options[:builder]
          args << {} unless args.last.is_a?(Hash)
          args.last[:builder] ||= options[:builder]
        end

        case record_or_name_or_array
        when String, Symbol
          if nested_attributes_association?(record_or_name_or_array)
            return fields_for_with_nested_attributes(record_or_name_or_array, args, block)
          else
            name = "#{object_name}#{index}[#{record_or_name_or_array}]"
          end
        when Array
          object = record_or_name_or_array.last
          name = "#{object_name}#{index}[#{ActiveModel::Naming.singular(object)}]"
          args.unshift(object)
        else
          object = record_or_name_or_array
          name = "#{object_name}#{index}[#{ActiveModel::Naming.singular(object)}]"
          args.unshift(object)
        end

        @template.fields_for(name, *args, &block)
      end