module BigRecord::Callbacks

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 BigRecord::Base#save call:

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 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 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 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 are called for each object found and instantiated by a finder, such as BigRecord::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 BigRecord::Base#save will return false. If BigRecord::Base#save! is called it will raise a RecordNotSave error. Nothing will be appended to the errors object.

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.

Constants

CALLBACKS

Public Instance Methods

after_create() click to toggle source

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

# File lib/big_record/callbacks.rb, line 251
def after_create() end
after_destroy() click to toggle source

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

class Contact < ActiveRecord::Base
  after_destroy { |record| logger.info( "Contact #{record.id} was destroyed." ) }
end
# File lib/big_record/callbacks.rb, line 318
def after_destroy()  end
after_save() click to toggle source

Is called after BigRecord::Base#save (regardless of whether it's a create or update save).

class Contact < ActiveRecord::Base
  after_save { logger.info( 'New contact saved!' ) }
end
# File lib/big_record/callbacks.rb, line 239
def after_save()  end
after_update() click to toggle source

Is called after BigRecord::Base#save on existing objects that have a record.

# File lib/big_record/callbacks.rb, line 263
def after_update() end
after_validation() click to toggle source

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

# File lib/big_record/callbacks.rb, line 276
def after_validation() end
after_validation_on_create() click to toggle source

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

# File lib/big_record/callbacks.rb, line 284
def after_validation_on_create()  end
after_validation_on_update() click to toggle source

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

# File lib/big_record/callbacks.rb, line 292
def after_validation_on_update()  end
before_create() click to toggle source

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

# File lib/big_record/callbacks.rb, line 248
def before_create() end
before_destroy() click to toggle source

Is called before BigRecord::Base#destroy.

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

# File lib/big_record/callbacks.rb, line 311
def before_destroy() end
before_save() click to toggle source

Is called before BigRecord::Base#save (regardless of whether it's a create or update save).

# File lib/big_record/callbacks.rb, line 232
def before_save() end
before_update() click to toggle source

Is called before BigRecord::Base#save on existing objects that have a record.

# File lib/big_record/callbacks.rb, line 260
def before_update() end
before_validation() click to toggle source

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

# File lib/big_record/callbacks.rb, line 273
def before_validation() end
before_validation_on_create() click to toggle source

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

# File lib/big_record/callbacks.rb, line 280
def before_validation_on_create() end
before_validation_on_update() click to toggle source

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

# File lib/big_record/callbacks.rb, line 288
def before_validation_on_update() end

Private Instance Methods

callback(method) click to toggle source
# File lib/big_record/callbacks.rb, line 327
def callback(method)
  notify(method)

  callbacks_for(method).each do |callback|
    result = case callback
      when Symbol
        self.send(callback)
      when String
        eval(callback, binding)
      when Proc, Method
        callback.call(self)
      else
        if callback.respond_to?(method)
          callback.send(method, self)
        else
          raise ActiveRecordError, "Callbacks must be a symbol denoting the method to call, a string to be evaluated, a block to be invoked, or an object responding to the callback method."
        end
    end
    return false if (result.is_a?FalseClass)
  end

  result = send(method) if respond_to_without_attributes?(method)

  return result
end
callbacks_for(method) click to toggle source
# File lib/big_record/callbacks.rb, line 353
def callbacks_for(method)
  self.class.read_inheritable_attribute(method.to_sym) or []
end
invoke_and_notify(method) click to toggle source
# File lib/big_record/callbacks.rb, line 357
def invoke_and_notify(method)
  notify(method)
  send(method) if respond_to_without_attributes?(method)
end