Validation 验证嵌套模型参数的唯一性

我有三种型号：

class User < ActiveRecord::Base
  has_many :user_countries
  accepts_nested_attributes_for :user_countries, reject_if: :all_blank
  validate :user_countries
end

class UserCountry < ActiveRecord::Base
  belongs_to :user
  belongs_to :country
end

class Country < ActiveRecord::Base
  has_many :user_countries
  has_many :users, :through => :user_countries
end

---

在

user\u国家/地区

型号中尝试此操作

class UserCountry < ActiveRecord::Base
  belongs_to :user
  belongs_to :country
  validates_uniqueness_of :user_id, scope: :country_id, allow_blank: false
end

---

谢谢！尽管正在引发ActiveRecord:：RecordNotUnique异常。。我如何拯救它？我已经添加了userCountry中的validates\u唯一性\u。。。

class UserCountry < ActiveRecord::Base
  belongs_to :user
  belongs_to :country
  validates_uniqueness_of :user_id, scope: :country_id, allow_blank: false
end

add_index :user_countries, [ :user_id, :country_id ], :unique => true