fields_for_with_nested_attributes

# File actionview/lib/action_view/helpers/form_helper.rb, line 1895
        def fields_for_with_nested_attributes(association_name, association, options, block)
          name = "#{object_name}[#{association_name}_attributes]"
          association = convert_to_model(association)

          if association.respond_to?(:persisted?)
            association = [association] if @object.send(association_name).respond_to?(:to_ary)
          elsif !association.respond_to?(:to_ary)
            association = @object.send(association_name)
          end

          if association.respond_to?(:to_ary)
            explicit_child_index = options[:child_index]
            output = ActiveSupport::SafeBuffer.new
            association.each do |child|
              options[:child_index] = nested_child_index(name) unless explicit_child_index
              output << fields_for_nested_model("#{name}[#{options[:child_index]}]", child, options, block)
            end
            output
          elsif association
            fields_for_nested_model(name, association, options, block)
          end
        end