fields_for_with_nested_attributes

# File actionpack/lib/action_view/helpers/form_helper.rb, line 1070
        def fields_for_with_nested_attributes(association_name, args, block)
          name = "#{object_name}[#{association_name}_attributes]"
          association = args.first

          if association.respond_to?(:new_record?)
            association = [association] if @object.send(association_name).is_a?(Array)
          elsif !association.is_a?(Array)
            association = @object.send(association_name)
          end

          if association.is_a?(Array)
            explicit_child_index = args.last[:child_index] if args.last.is_a?(Hash)
            association.map do |child|
              fields_for_nested_model("#{name}[#{explicit_child_index || nested_child_index(name)}]", child, args, block)
            end.join
          elsif association
            fields_for_nested_model(name, association, args, block)
          end
        end