Module | ActiveRecord::Callbacks
In: | lib/active_record/callbacks.rb

Callbacks are hooks into the lifecycle of an Active Record object that allow 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:

- (-) `save`
- (-) `valid`
- (1) `before_validation`
- (2) `before_validation_on_create`
- (-) `validate`
- (-) `validate_on_create`
- (3) `after_validation`
- (4) `after_validation_on_create`
- (5) `before_save`
- (6) `before_create`
- (-) `create`
- (7) `after_create`
- (8) `after_save`

That‘s a total of eight 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` are 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 use 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 are sometimes 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 quotes (’) 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` are called for each object found and instantiated 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.

## `before_validation*` returning statements

If the returning value of a `before_validation` callback can be evaluated to `false`, the process will be aborted and `Base#save` will return `false`. If Base#save! is called it will raise a RecordNotSaved exception. Nothing will be appended to the errors object.

## Canceling 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

after_create after_destroy after_save after_update after_validation after_validation_on_create after_validation_on_update before_create before_destroy before_save before_update before_validation before_validation_on_create before_validation_on_update

### 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 )

### Public Instance methods

after_create()

Is called after `Base.save` on new objects that haven‘t been saved yet (no record exists).

after_destroy()

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

     class Contact < ActiveRecord::Base
       after_destroy { |record| logger.info( "Contact #{record.id} was destroyed." ) }
     end

after_save()

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

     class Contact < ActiveRecord::Base
       after_save { logger.info( 'New contact saved!' ) }
     end

after_update()

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

after_validation()

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

after_validation_on_create()

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).

after_validation_on_update()

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

before_create()

Is called before `Base.save` on new objects that haven‘t been saved yet (no record exists).

before_destroy()

Is called before `Base.destroy`.

Note: If you need to destroy or nullify associated records first, use the `:dependent` option on your associations.

before_save()

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

before_update()

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

before_validation()

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

before_validation_on_create()

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).

before_validation_on_update()

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

[Validate]