Ruby on rails Mongoid字段自动求和

各位!！我有一个模型：

class Model
  include Mongoid::Document

field :price1, :type =>Integer 
field :price2, :type =>Integer 
field :price3, :type =>Integer <== I want this field to be always result of price1 + price2
 end

类模型
include Mongoid:：Document
字段：price1，：type=>Integer
字段：price2，：type=>Integer
字段：price3，：type=>Integer您要使用该接口。公开了各种回调，例如：

require "mongoid"
require "pp"

Mongoid.configure.connect_to("test")

class Model
  include Mongoid::Document

  field :price1, type: Integer
  field :price2, type: Integer
  field :price3, type: Integer

  store_in collection: "mymodel"

  before_save do |document|
    document.price3 = document.price1 + document.price2
  end

end



model = Model.new

model.price1 = 2
model.price2 = 3

model.save

这将导致“price3”字段被设置为其他两个值的总和