fields_for_with_nested_attributes

# File actionview/lib/action_view/helpers/form_helper.rb, line 2703
        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.public_send(association_name).respond_to?(:to_ary)
          elsif !association.respond_to?(:to_ary)
            association = @object.public_send(association_name)
          end

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