Callbacks are methods that get invoked at various stages of an Activerecord objects life cycle. and thus it is possible to write code that will run whenever an Activerecord object is created, saved, updated, deleted, validated, or loaded from the database.

Below is the list of the callbacks that are available in rails framework:

Creating an Object

• before_validation ,after_validation, before_save ,around_save, before_create ,around_create, after_create, after_save

Updating an Object

• before_validation ,after_validation, before_save ,around_save, before_update ,around_update, after_update ,after_save

Destroying an Object

• before_destroy, around_destroy, after_destroy

Some examples of callback use:

• before validation:

  class User < ActiveRecord::Base validates :name, :first_name ,:last_name , :presence => true

  before_validation :ensure_user_has_a_name

  protected def ensure_user_has_a_name self.name = first_name + + last_name unless first_name.nil && last_name.nil

  end

  end

• before_save:

  class Location < ActiveRecord::Base

  validates :street_name, :city, :zip, :state, :country , :presence => true

  before_save :ensure_address

  protected

  def ensure_address self.address = [street_name , city , zip , state , country].compact.join(,)

  end

  end

before_save is a callback that is invoked both while saving new record or updating existing record.

There are some methods which do not trigger callbacks . They are :

• decrement, decrement_counter, delete, delete_all, find_by_sql, increment, increment_counter, toggle, touch, update_column, update_all, update_counters

Conditional Callbacks:

We can make the execution of a callback method conditional on the satisfaction of a given condition. We can do this using the :if and :unless options, which can take a symbol, a string or a Proc. You may use the :if option when you want to specify under which conditions the callback should be called. If you want to specify the conditions under which the callback should not be called, then you may use the :unless option.For example:

class Device < ActiveRecord::Base

  before_save :check_gateway

  def check_gateway
   self.gateway = 'gateway.sandbox.push.apple.com' if file_path.present? && gateway.blank?
  end
end

Using proc as condition for callback.

 class Order < ActiveRecord::Base
    before_save :normalize_card_number,:if => Proc.new { |order| order.paid_with_card? }
 end