Module ActiveRecord::Callbacks
In: vendor/rails/activerecord/lib/active_record/callbacks.rb

Callbacks are hooks into the lifecycle of an Active Record object that allows you to trigger logic before or after an alteration of the object state. This can be used to make sure that associated and dependent objects are deleted when destroy is called (by overwriting before_destroy) or to massage attributes before they’re validated (by overwriting before_validation). As an example of the callbacks initiated, consider the Base#save call:

That’s a total of nine callbacks, which gives you immense power to react and prepare for each state in the Active Record lifecycle.

Examples:

  class CreditCard < ActiveRecord::Base
    # Strip everything but digits, so the user can specify "555 234 34" or
    # "5552-3434" or both will mean "55523434"
    def before_validation_on_create
      self.number = number.gsub(/[^0-9]/, "") if attribute_present?("number")
    end
  end

  class Subscription < ActiveRecord::Base
    before_create :record_signup

    private
      def record_signup
        self.signed_up_on = Date.today
      end
  end

  class Firm < ActiveRecord::Base
    # Destroys the associated clients and people when the firm is destroyed
    before_destroy { |record| Person.destroy_all "firm_id = #{record.id}"   }
    before_destroy { |record| Client.destroy_all "client_of = #{record.id}" }
  end

Inheritable callback queues

Besides the overwriteable callback methods, it’s also possible to register callbacks through the use of the callback macros. Their main advantage is that the macros add behavior into a callback queue that is kept intact down through an inheritance hierarchy. Example:

  class Topic < ActiveRecord::Base
    before_destroy :destroy_author
  end

  class Reply < Topic
    before_destroy :destroy_readers
  end

Now, when Topic#destroy is run only destroy_author is called. When Reply#destroy is run both destroy_author and destroy_readers is called. Contrast this to the situation where we’ve implemented the save behavior through overwriteable methods:

  class Topic < ActiveRecord::Base
    def before_destroy() destroy_author end
  end

  class Reply < Topic
    def before_destroy() destroy_readers end
  end

In that case, Reply#destroy would only run destroy_readers and not destroy_author. So use the callback macros when you want to ensure that a certain callback is called for the entire hierarchy and the regular overwriteable methods when you want to leave it up to each descendent to decide whether they want to call super and trigger the inherited callbacks.

*IMPORTANT:* In order for inheritance to work for the callback queues, you must specify the callbacks before specifying the associations. Otherwise, you might trigger the loading of a child before the parent has registered the callbacks and they won’t be inherited.

Types of callbacks

There are four types of callbacks accepted by the callback macros: Method references (symbol), callback objects, inline methods (using a proc), and inline eval methods (using a string). Method references and callback objects are the recommended approaches, inline methods using a proc is some times appropriate (such as for creating mix-ins), and inline eval methods are deprecated.

The method reference callbacks work by specifying a protected or private method available in the object, like this:

  class Topic < ActiveRecord::Base
    before_destroy :delete_parents

    private
      def delete_parents
        self.class.delete_all "parent_id = #{id}"
      end
  end

The callback objects have methods named after the callback called with the record as the only parameter, such as:

  class BankAccount < ActiveRecord::Base
    before_save      EncryptionWrapper.new("credit_card_number")
    after_save       EncryptionWrapper.new("credit_card_number")
    after_initialize EncryptionWrapper.new("credit_card_number")
  end

  class EncryptionWrapper
    def initialize(attribute)
      @attribute = attribute
    end

    def before_save(record)
      record.credit_card_number = encrypt(record.credit_card_number)
    end

    def after_save(record)
      record.credit_card_number = decrypt(record.credit_card_number)
    end

    alias_method :after_find, :after_save

    private
      def encrypt(value)
        # Secrecy is committed
      end

      def decrypt(value)
        # Secrecy is unveiled
      end
  end

So you specify the object you want messaged on a given callback. When that callback is triggered, the object has a method by the name of the callback messaged.

The callback macros usually accept a symbol for the method they’re supposed to run, but you can also pass a "method string", which will then be evaluated within the binding of the callback. Example:

  class Topic < ActiveRecord::Base
    before_destroy 'self.class.delete_all "parent_id = #{id}"'
  end

Notice that single plings (’) are used so the #{id} part isn’t evaluated until the callback is triggered. Also note that these inline callbacks can be stacked just like the regular ones:

  class Topic < ActiveRecord::Base
    before_destroy 'self.class.delete_all "parent_id = #{id}"',
                   'puts "Evaluated after parents are destroyed"'
  end

The after_find and after_initialize exceptions

Because after_find and after_initialize is called for each object instantiated found by a finder, such as Base.find(:all), we’ve had to implement a simple performance constraint (50% more speed on a simple test case). Unlike all the other callbacks, after_find and after_initialize will only be run if an explicit implementation is defined (def after_find). In that case, all of the callback types will be called.

Cancelling callbacks

If a before_* callback returns false, all the later callbacks and the associated action are cancelled. If an after_* callback returns false, all the later callbacks are cancelled. Callbacks are generally run in the order they are defined, with the exception of callbacks defined as methods on the model, which are called last.

Methods

Included Modules

Observable

Constants

CALLBACKS = %w( after_find after_initialize before_save after_save before_create after_create before_update after_update before_validation after_validation before_validation_on_create after_validation_on_create before_validation_on_update after_validation_on_update before_destroy after_destroy )

External Aliases

instantiate -> instantiate_without_callbacks
instantiate_with_callbacks -> instantiate

Public Instance methods

Is called after Base.save on new objects that haven’t been saved yet (no record exists).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 259
259:     def after_create() end

Is called after Base.destroy (and all the attributes have been frozen).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 319
319:     def after_destroy()  end

Is called after Base.save (regardless of whether it’s a create or update save).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 247
247:     def after_save()  end

Is called after Base.save on existing objects that has a record.

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 271
271:     def after_update() end

Is called after Validations.validate (which is part of the Base.save call).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 284
284:     def after_validation() end

Is called after Validations.validate (which is part of the Base.save call) on new objects that haven’t been saved yet (no record exists).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 292
292:     def after_validation_on_create()  end

Is called after Validations.validate (which is part of the Base.save call) on existing objects that has a record.

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 300
300:     def after_validation_on_update()  end

Is called before Base.save on new objects that haven’t been saved yet (no record exists).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 256
256:     def before_create() end

Is called before Base.destroy.

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 316
316:     def before_destroy() end

Is called before Base.save (regardless of whether it’s a create or update save).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 244
244:     def before_save() end

Is called before Base.save on existing objects that has a record.

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 268
268:     def before_update() end

Is called before Validations.validate (which is part of the Base.save call).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 281
281:     def before_validation() end

Is called before Validations.validate (which is part of the Base.save call) on new objects that haven’t been saved yet (no record exists).

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 288
288:     def before_validation_on_create() end

Is called before Validations.validate (which is part of the Base.save call) on existing objects that has a record.

[Source]

     # File vendor/rails/activerecord/lib/active_record/callbacks.rb, line 296
296:     def before_validation_on_update() end

[Validate]