require 'observer' module ActiveRecord # 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: # # * (-) save # * (-) valid # * (1) before_validation # * (2) before_validation_on_create # * (-) validate # * (-) validate_on_create # * (3) after_validation # * (4) after_validation_on_create # * (5) before_save # * (6) before_create # * (-) create # * (7) after_create # * (8) after_save # # 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+ 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. # # == 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 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 Base#save will return +false+. # If Base#save! is called it will raise a +RecordNotSave+ exception. # 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. module Callbacks 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 ) def self.included(base) #:nodoc: base.extend Observable [:create_or_update, :valid?, :create, :update, :destroy].each do |method| base.send :alias_method_chain, method, :callbacks end CALLBACKS.each do |method| base.class_eval <<-"end_eval" def self.#{method}(*callbacks, &block) callbacks << block if block_given? write_inheritable_array(#{method.to_sym.inspect}, callbacks) end end_eval end end # Is called when the object was instantiated by one of the finders, like Base.find. #def after_find() end # Is called after the object has been instantiated by a call to Base.new. #def after_initialize() end # Is called _before_ Base.save (regardless of whether it's a +create+ or +update+ save). def before_save() 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 def after_save() end def create_or_update_with_callbacks #:nodoc: return false if callback(:before_save) == false result = create_or_update_without_callbacks callback(:after_save) result end private :create_or_update_with_callbacks # Is called _before_ Base.save on new objects that haven't been saved yet (no record exists). def before_create() end # Is called _after_ Base.save on new objects that haven't been saved yet (no record exists). def after_create() end def create_with_callbacks #:nodoc: return false if callback(:before_create) == false result = create_without_callbacks callback(:after_create) result end private :create_with_callbacks # Is called _before_ Base.save on existing objects that have a record. def before_update() end # Is called _after_ Base.save on existing objects that have a record. def after_update() end def update_with_callbacks #:nodoc: return false if callback(:before_update) == false result = update_without_callbacks callback(:after_update) result end private :update_with_callbacks # Is called _before_ Validations.validate (which is part of the Base.save call). def before_validation() end # Is called _after_ Validations.validate (which is part of the Base.save call). def after_validation() end # 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). def before_validation_on_create() 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). def after_validation_on_create() end # Is called _before_ Validations.validate (which is part of the Base.save call) on # existing objects that have a record. def before_validation_on_update() end # Is called _after_ Validations.validate (which is part of the Base.save call) on # existing objects that have a record. def after_validation_on_update() end def valid_with_callbacks? #:nodoc: return false if callback(:before_validation) == false if new_record? then result = callback(:before_validation_on_create) else result = callback(:before_validation_on_update) end return false if result == false result = valid_without_callbacks? callback(:after_validation) if new_record? then callback(:after_validation_on_create) else callback(:after_validation_on_update) end return result end # Is called _before_ Base.destroy. # # Note: If you need to _destroy_ or _nullify_ associated records first, # use the :dependent option on your associations. def before_destroy() end # 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 def after_destroy() end def destroy_with_callbacks #:nodoc: return false if callback(:before_destroy) == false result = destroy_without_callbacks callback(:after_destroy) result end private 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 == false end result = send(method) if respond_to_without_attributes?(method) return result end def callbacks_for(method) self.class.read_inheritable_attribute(method.to_sym) or [] end def invoke_and_notify(method) notify(method) send(method) if respond_to_without_attributes?(method) end def notify(method) #:nodoc: self.class.changed self.class.notify_observers(method, self) end end end