如何使用"validates:attribute"来引用父对象属性的值来验证属性？

Question

在O'Reilly的"Head first Rails"(编辑,2009)的练习中,有2个相关的对象.

在"飞行"的对象[我用来注释宝石显示每个属性]:

# == Schema Information
#
# Table name: flights
#
#  id                :integer         not null, primary key
#  departure         :datetime
#  arrival           :datetime
#  destination       :string(255)
#  baggage_allowance :decimal(, )
#  capacity          :integer
#  created_at        :datetime
#  updated_at        :datetime
#

class Flight < ActiveRecord::Base
  has_many :seats
end

Ant "Seat"对象:

# == Schema Information
#
# Table name: seats
#
#  id         :integer         not null, primary key
#  flight_id  :integer
#  name       :string(255)
#  baggage    :decimal(, )
#  created_at :datetime
#  updated_at :datetime
#

class Seat < ActiveRecord::Base
  belongs_to :flight
end

您可能猜测seat.baggage值应始终小于或等于seat.flight.baggage_allowance.

所以我写了这个验证器,效果很好:

class Seat < ActiveRecord::Base

  belongs_to :flight

  def validate
      if baggage > flight.baggage_allowance
      errors.add_to_base("Your have to much baggage for this flight!")
    end 
  end  
end

然后我试着用这个更漂亮的那个重构它:

      validates :baggage, :numericality => { :less_than_or_equal_to => flight.baggage_allowance }, :presence => true 

但它导致SeatsController中的NameError:

undefined local variable or method `flight' for #<Class:0x68ac3d8>"

比我还试过"self.flight.baggage_allowance":

validates :baggage, :numericality => { :less_than_or_equal_to => self.flight.baggage_allowance }, :presence => true

但它会引发NoMethodError异常:

undefined method `flight' for #<Class:0x67e9b40>

有没有办法让更漂亮的验证器工作？

进行此类验证的最佳做法是哪种？

谢谢.

- -编辑 - -

正如毛里西奥·林哈里斯(MaurícioLinhares)此后提出的那样,问题是可以解决的:定义:bagging_allowance符号.我想更好地了解真正需要自定义验证的地方.这个怎么样,是否有可能将此转换为"验证"方法？为什么？

class Seat < ActiveRecord::Base
.
.
.
def validate
  if flight.capacity <= flight.seats.size
    errors.add_to_base("The flight is fully booked, no more seats available!")
  end
end

再次感谢你.

Answer 1

你可以这样做:

class Seat < ActiveRecord::Base

  validates :baggage, :numericality => { :less_than_or_equal_to => :baggage_allowance }, :presence => true 

  belongs_to :flight

  def baggage_allowance
    flight.baggage_allowance
  end  
end

编辑

你不能这样做:

class Seat < ActiveRecord::Base
        validates :baggage, :numericality => { :less_than_or_equal_to => flight.baggage_allowance }, :presence => true 
end

因为方法验证是在类级别调用的,所以没有可用的飞行变量,因为它是一个实例变量.当您使用:baggage_allowance对其进行配置时,您可以告诉Rails在Seat实例上调用:baggage_allowance方法以便能够访问该值.