ruby-on-rails - 如何限制在nested_form_for 中创建的关联中未删除项目的数量，使用link_to_add 来建立关联？

Question

我正在使用 rails 4.0.0 并且我有一个表单，我想限制为 3 个添加的关联。最干净的方法是什么？

= nested_form_for(@donation) do |f|
  = render 'shared/error_messages', object: f.object

  = f.fields_for :donation_devices do |f2|
    %p
      = f2.collection_select(:device_model_id, DeviceModel.all, :id, :name)
      = f2.link_to_remove "Remove Device"
  = f.link_to_add "Add Device", :donation_devices