module ActiveRecord::Callbacks - activerecord-4.2.5.2 Documentation

# module ActiveRecord::Callbacks

# Active Record Callbacks¶ ↑

Callbacks are hooks into the life cycle 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 for a new record:

- (-) `save`

- (-) `valid`

- (1) `before_validation`

- (-) `validate`

- (2) `after_validation`

- (3) `before_save`

- (4) `before_create`

- (-) `create`

- (5) `after_create`

- (6) `after_save`

- (7) `after_commit`

Also, an `after_rollback` callback can be configured to be triggered whenever a rollback is issued. Check out `ActiveRecord::Transactions` for more details about `after_commit` and `after_rollback`.

Additionally, an `after_touch` callback is triggered whenever an object is touched.

Lastly an `after_find` and `after_initialize` callback is triggered for each object that is found and instantiated by a finder, with `after_initialize` being triggered after new objects are instantiated as well.

There are nineteen callbacks in total, which give you immense power to react and prepare for each state in the Active Record life cycle. The sequence for calling `Base#save` for an existing record is similar, except that each `_create` callback is replaced by the corresponding `_update` callback.

Examples:

    class CreditCard < ActiveRecord::Base
      # Strip everything but digits, so the user can specify "555 234 34" or
      # "5552-3434" and both will mean "55523434"
      before_validation(on: :create) do
        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 overwritable 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.

    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 following situation where the `before_destroy` method is overridden:

    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 overwritable methods when you want to leave it up to each descendant 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
      after_save       EncryptionWrapper.new
      after_initialize EncryptionWrapper.new
    end

    class EncryptionWrapper
      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_initialize, :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. You can make these callbacks more flexible by passing in other initialization data such as the name of the attribute to work with:

    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.send("#{@attribute}=", encrypt(record.send("#{@attribute}")))
      end

      def after_save(record)
        record.send("#{@attribute}=", decrypt(record.send("#{@attribute}")))
      end

      alias_method :after_initialize, :after_save

      private
        def encrypt(value)
          # Secrecy is committed
        end

        def decrypt(value)
          # Secrecy is unveiled
        end
    end

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

## `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 ActiveRecord::Validations#save! is called it will raise a ActiveRecord::RecordInvalid 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. 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.

## Ordering callbacks¶ ↑

Sometimes the code needs that the callbacks execute in a specific order. For example, a `before_destroy` callback (`log_children` in this case) should be executed before the children get destroyed by the +dependent: destroy+ option.

Let's look at the code below:

    class Topic < ActiveRecord::Base
      has_many :children, dependent: destroy

      before_destroy :log_children

      private
        def log_children
          # Child processing
        end
    end

In this case, the problem is that when the `before_destroy` callback is executed, the children are not available because the `destroy` callback gets executed first. You can use the `prepend` option on the `before_destroy` callback to avoid this.

    class Topic < ActiveRecord::Base
      has_many :children, dependent: destroy

      before_destroy :log_children, prepend: true

      private
        def log_children
          # Child processing
        end
    end

This way, the `before_destroy` gets executed before the `dependent: destroy` is called, and the data is still available.

## Transactions¶ ↑

The entire callback chain of a `save`, `save!`, or `destroy` call runs within a transaction. That includes `after_*` hooks. If everything goes fine a COMMIT is executed once the chain has been completed.

If a `before_*` callback cancels the action a ROLLBACK is issued. You can also trigger a ROLLBACK raising an exception in any of the callbacks, including `after_*` hooks. Note, however, that in that case the client needs to be aware of it because an ordinary `save` will raise such exception instead of quietly returning `false`.

## Debugging callbacks¶ ↑

The callback chain is accessible via the `_*_callbacks` method on an object. ActiveModel Callbacks support `:before`, `:after` and `:around` as values for the `kind` property. The `kind` property defines what part of the chain the callback runs in.

To find all callbacks in the before_save callback chain:

    Topic._save_callbacks.select { |cb| cb.kind.eql?(:before) }

Returns an array of callback objects that form the before_save chain.

To further check if the before_save chain contains a proc defined as `rest_when_dead` use the `filter` property of the callback object:

    Topic._save_callbacks.select { |cb| cb.kind.eql?(:before) }.collect(&:filter).include?(:rest_when_dead)

Returns true or false depending on whether the proc is contained in the before_save callback chain on a Topic model.

### Constants

CALLBACKS