Mongoid单表继承不实例化子类

Question

我有一个从PaymentMethod继承的PaymentPayPal类

class PaymentMethod
  include Mongoid::Document
  embedded_in :store

  field :method, type: String
end

class PaymentKhipu < PaymentMethod
  field :receiver_id, type: String
  field :secret,      type: String
end

但每次我想为商店建立付款方式我都会得到相同的父类

class Store
  include Mongoid::Document
  embeds_one :payment_method
end

s.build_payment_method({}, PaymentPayPal)
=> #<PaymentMethod _id: 529db89f24f1e727db000001, method: nil, _type: "PaymentMethod">

我可以手动创建类，但我想通过关联

创建它

PaymentPayPal.new
=> #<PaymentPayPal _id: 529dba3324f1e727db000002, method: "paypal", _type: "PaymentPayPal", receiver_id: nil, secret: nil>

我正在使用mongoid 4.0和rails 4.0