Ruby on rails RubyonRails-保存前获取多态\u id

我有两个模型：

多态性

class Custo < ActiveRecord::Base
  belongs_to :custavel, polymorphic: true
  accepts_nested_attributes_for :custavel, allow_destroy: true
  validates_presence_of :numero, :serie
  validates_uniqueness_of :numero, scope: :serie
end

---

但它在验证时失败，因为custavel\u id还不存在。如何操作？

修改

回调方法

before_save :set_numero

def set_numero
  write_attribute(:numero, custavel_id) if custavel_type == 'Pagamento'
end

before_save :set_numero
def set_numero
  return unless custavel_type == 'Pagamento'
  write_attribute(:numero, custavel_id)
end

before_save :set_numero

def set_numero
  write_attribute(:numero, custavel_id) if custavel_type == 'Pagamento'
end