Determine what attributes were changed in Rails after_save callback?

Question

I'm setting up an after_save callback in my model observer to send a notification only if the model's published attribute was changed from false to true. Since methods such as changed? are only useful before the model is saved, the way I'm currently (and unsuccessfully) trying to do so is as follows:

def before_save(blog)
  @og_published = blog.published?
end

def after_save(blog)
  if @og_published == false and blog.published? == true
    Notification.send(...)
  end
end

Does anyone have any suggestions as to the best way to handle this, preferably using model observer callbacks (so as not to pollute my controller code)?

Answer 1

Rails 5.1+

Use saved_change_to_published?:

class SomeModel < ActiveRecord::Base
  after_update :send_notification_after_change

  def send_notification_after_change
    Notification.send(…) if (saved_change_to_published? && self.published == true)
  end

end

Or if you prefer, saved_change_to_attribute?(:published).

Rails 3–5.1

Warning

This approach works through Rails 5.1 (but is deprecated in 5.1 and has breaking changes in 5.2). You can read about the change in this pull request.

In your after_update filter on the model you can use _changed? accessor. So for example:

class SomeModel < ActiveRecord::Base
  after_update :send_notification_after_change

  def send_notification_after_change
    Notification.send(...) if (self.published_changed? && self.published == true)
  end

end

It just works.