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

try:

def attribute_changed(attribute_name, old_value, new_value)
end

def attribute=(attribute_name, value)
  returning(super) do
    attribute_changed(attribute_name, attribute_was(attribute_name), attribute(attribute_name))
  end
end

Just invented this now, but it should work.

Answer 2

You could always access the private method changed_attributes and check the keys there using a before_save and do with it what you will.

Answer 3

The easiest way that I found:

after_save :my_method, :if => Proc.new{ self.my_attribute_changed? }

def my_method
  ... do stuff...
end

Answer 4

For Rails 4

def attribute=(value)
  super(value)
  your_callback(self.attribute)
end

If you want to overwrite the value of attribute you should use write_attribute(:attribute, your_callback(self.attribute)) and not attribute= or you'll loop over it until you get a stack too deep exception.

Answer 5

cwninja's answer should do the trick, but there is a little bit more to it.

First of all, the base attribute handling is done with the write_attribute method so you should be tapping into this.

Rails also does have a built in callback structure which could be nice to tap into though it doesn't allow for passing arguments which is a bit of an annoyance.

Using custom callbacks you could do it like this:

class Person < ActiveRecord::Base

  def write_attribute(attr_name, value)
    attribute_changed(attr_name, read_attribute(attr_name), value)
    super
  end

  private

    def attribute_changed(attr, old_val, new_val)
      logger.info "Attribute Changed: #{attr} from #{old_val} to #{new_val}"
    end

 end

If you wanted to try to use Rails callbacks (especially useful if you might have multiple callbacks and/or subclassing) you could do something like this:

class Person < ActiveRecord::Base
  define_callbacks :attribute_changed

  attribute_changed :notify_of_attribute_change

  def write_attribute(attr_name, value)
    returning(super) do
      @last_changed_attr = attr_name
      run_callbacks(:attribute_changed)
    end
  end

  private

    def notify_of_attribute_change
      attr = @last_changed_attr
      old_val, new_val = send("#{attr}_change")
      logger.info "Attribute Changed: #{attr} from #{old_val} to #{new_val}"
    end

end

Answer 6

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.