Add save callback to a single ActiveRecord instance, is it possible?

Question

Is it possible to add a callback to a single ActiveRecord instance? As a further constraint this is to go on a library so I don't have control over the class (except to monkey-patch it).

This is more or less what I want to do:

def do_something_creazy
  message = Message.new
  message.on_save_call :do_even_more_crazy_stuff
end

def do_even_more_crazy_stuff(message)
  puts "Message #{message} has been saved! Hallelujah!"
end

Answer 1

You could do something like that by adding a callback to the object right after creating it and like you said, monkey-patching the default AR before_save method:

def do_something_ballsy
    msg = Message.new
    def msg.before_save(msg)
        puts "Message #{msg} is saved."
        # Calls before_save defined in the model
        super
    end
end

Answer 2

For something like this you can always define your own crazy handlers:

class Something < ActiveRecord::Base
  before_save :run_before_save_callbacks

  def before_save(&block)
    @before_save_callbacks ||= [ ]
    @before_save_callbacks << block
  end

protected
  def run_before_save_callbacks
    return unless @before_save_callbacks

    @before_save_callbacks.each do |callback|
      callback.call
    end
  end
end

This could be made more generic, or an ActiveRecord::Base extension, whatever suits your problem scope. Using it should be easy:

something = Something.new

something.before_save do
  Rails.logger.warn("I'm saving!")
end