Ruby on Rails/ActiveRecord/Callbacks

Callbacks Callbacks provide a means of hooking into an ActiveRecord object's lifecycle. Implementing Callbacks There are four types of callbacks accepted by the callback macros: Method references (symbol) Callback objects Inline methods (using a proc) Inline eval methods (using a string) - deprecated. 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. Method Reference 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 Callback Objects 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. Proc Example of using a Proc for a callback: class Person before_save Proc.new { |model| model.do_something } end Inline Eval 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 Callback Reference before_save This method is called before an ActiveRecord object is saved. after_save Once the active record object saved some method will be fired in that scenario we have to use the after_save callback. before_create called before creating a new object of the model after_create Called after creating new object and just before saving the records 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 Partially Documented Callbacks The following callbacks are partially documented. Their use is discouraged because of "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." performance issues. after_find The after_find callback is only executed if there is an explicit implementation in the model class. It will be called for each object returned from a find, and thus can potentially affect performance as noted in the Rails API Documentation. after_initialize The after_initialize method is only executed if there is an explicit implementation in the model class. It will be called whenever an ActiveRecord model is initialized. It will be called for each object returned from a find, and thus can potentially affect performance as noted in the Rails API documentation. References =