Rails - 多个嵌套对象验证

Question

我有以下课程：

account
 has_many :orders
 has_many :payment_methods

order
 belongs_to :account
 has_one :payment

payment
 belongs_to :order
 belongs_to :payment_method

payment_method
 belongs_to :account
 has_many :payments

帐户有订单和付款方式。付款是订单和付款方式的结合。我尝试对付款进行验证，以确保付款方式属于下订单的帐户。

payment
  validate :payment_method_belongs_to_account

  def payment_method_belongs_to_account
    if payment_method.account != order.account
      # add error
    end
  end

我遇到的麻烦是当我在订单下创建付款作为嵌套对象时：

account = Account.new
order = Order.new(account: account)
payment_method = PaymentMethod.new(account: account)
order.payment.build(payment_method: payment_method)

if order.payment.valid?
  # I never get here
end

通过在订单下将付款创建为嵌套对象，payment.order为零，因此payment.order.account失败。

如何解决此问题？