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. 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.

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.