Mongoid Single table inheritance not instantiating the subclass

Question

I have a PaymentPayPal class that inherits from PaymentMethod

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

But everytime i want to build a payment method for an store i get the same parent class

class Store
  include Mongoid::Document
  embeds_one :payment_method
end

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

I can create the class manually, but i'll like to create it through the association

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

Im using mongoid 4.0 and rails 4.0