Callback for changed ActiveRecord attributes?

Question

I am aware of ActiveRecord::Dirty and the related methods, but I don\'t see a means by which I can subscribe to an attribute changed event. Something like:

Answer 1

For Rails 3:

class MyModel < ActiveRecord::Base
  after_update :my_listener, :if => :my_attribute_changed?

  def my_listener
    puts "Attribute 'my_attribute' has changed"
  end
end

Commented in: https://stackoverflow.com/a/1709956/316700

I don't find official documentation about this.