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:
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
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.
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
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.
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.
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.
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 ) |
Is called after Base.save on new objects that haven‘t been saved yet (no record exists).
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
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
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).
Is called after Validations.validate (which is part of the Base.save call) on existing objects that have a record.
Is called before Base.save on new objects that haven‘t been saved yet (no record exists).
Is called before Base.destroy.
Note: If you need to destroy or nullify associated records first, use the :dependent option on your associations.
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).
Is called before Validations.validate (which is part of the Base.save call) on existing objects that have a record.