fields_for_nested_model

# File actionpack/lib/action_view/helpers/form_helper.rb, line 1278
        def fields_for_nested_model(name, object, options, block)
          object = object.to_model if object.respond_to?(:to_model)

          if object.persisted?
            @template.fields_for(name, object, options) do |builder|
              block.call(builder)
              @template.concat builder.hidden_field(:id) unless builder.emitted_hidden_id?
            end
          else
            @template.fields_for(name, object, options, &block)
          end
        end