Rails —如何从after_save回调中查看新旧模型的值？

Question

Let's say I have the following models:

class V < ActiveRecord::Base
  belongs_to :p
  after_save :after_save_v
  ...
end

class P < ActiveRecord::Base
  has_many :vs
  # Has an attribute called "score" (defined in the DB table)
  ...
end

Creating a V increases vpscore by 1. Now in my after_save_v , I would like to track the old value of p.score . How can I do this?

Using a p.score_was does not work.

class V < ActiveRecord::Base
  def after_save_v
    puts p.score_was # => 1
    puts p.score # => 1
  end
end

What am I missing here? Or is this something that is not possible from the after_save callback?

Answer 1

In after_save , if you use the changes method, it returns the hash of all the changes on your object. You can specify which attribute's changes you need. This will be an array. [old-value, new-value]

In your Example, it can be self.p.changes["score"] . This returns an array [0, 1] , where 0 is the old value, and 1 is the new value