example of what a callback is using active record

callback_example.rb

class User < ActiveRecord::Base
  
  # here we set tha callback to be executed before saving the user
  before_save :create_username
  
  def create_username
    if username.blank?
      self.username = "example username"
    end
  end
end


user = User.new # first we instantiate the new user

# then, we save the user
# this is the moment when the callback gets executed
user.save