module ActiveRecord # = 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. # # 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. # # That's a total of twelve callbacks, which gives 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 overwriteable 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_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. 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_find, :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 Base#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. # module Callbacks extend ActiveSupport::Concern CALLBACKS = [ :after_initialize, :after_find, :after_touch, :before_validation, :after_validation, :before_save, :around_save, :after_save, :before_create, :around_create, :after_create, :before_update, :around_update, :after_update, :before_destroy, :around_destroy, :after_destroy, :after_commit, :after_rollback ] module ClassMethods include ActiveModel::Callbacks end included do include ActiveModel::Validations::Callbacks define_model_callbacks :initialize, :find, :touch, :only => :after define_model_callbacks :save, :create, :update, :destroy end def destroy #:nodoc: run_callbacks(:destroy) { super } end def touch(*) #:nodoc: run_callbacks(:touch) { super } end private def create_or_update #:nodoc: run_callbacks(:save) { super } end def create #:nodoc: run_callbacks(:create) { super } end def update(*) #:nodoc: run_callbacks(:update) { super } end end end