Way 1

dj.rb

class User < ActiveRecord::Base
  after_create :notify_user_created

  def notify_user_created
    User.delay.async_notify_user_created(self.id)
  end
  
  def self.async_notify_user_created(id)
    user = User.find(id)
    user.do_the_think_you_were_going_to_do
  end
end