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 <tt>Base#save</tt> call for a new record:
#
# * (-) <tt>save</tt>
# * (-) <tt>valid</tt>
# * (1) <tt>before_validation</tt>
# * (-) <tt>validate</tt>
# * (2) <tt>after_validation</tt>
# * (3) <tt>before_save</tt>
# * (4) <tt>before_create</tt>
# * (-) <tt>create</tt>
# * (5) <tt>after_create</tt>
# * (6) <tt>after_save</tt>
# * (7) <tt>after_commit</tt>
#
# Also, an <tt>after_rollback</tt> callback can be configured to be triggered whenever a rollback is issued.
# Check out <tt>ActiveRecord::Transactions</tt> for more details about <tt>after_commit</tt> and
# <tt>after_rollback</tt>.
#
# Additionally, an <tt>after_touch</tt> callback is triggered whenever an
# object is touched.
#
# Lastly an <tt>after_find</tt> and <tt>after_initialize</tt> callback is triggered for each object that
# is found and instantiated by a finder, with <tt>after_initialize</tt> being triggered after new objects
# are instantiated as well.
#
# There are nineteen callbacks in total, which give you immense power to react and prepare for each state in the
# Active Record life cycle. The sequence for calling <tt>Base#save</tt> for an existing record is similar,
# except that each <tt>_create</tt> callback is replaced by the corresponding <tt>_update</tt> 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 <tt>Topic#destroy</tt> is run only +destroy_author+ is called. When <tt>Reply#destroy</tt> 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, <tt>Reply#destroy</tt> 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 overwritable 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_initialize, :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_initialize, :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 <tt>#{id}</tt> 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
#
# == <tt>before_validation*</tt> returning statements
#
# If the returning value of a +before_validation+ callback can be evaluated to +false+, the process will be
# aborted and <tt>Base#save</tt> 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 <tt>before_*</tt> callback returns +false+, all the later callbacks and the associated action are
# cancelled. If an <tt>after_*</tt> 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 <tt>dependent: destroy</tt> is called, and the data is still available.
#
# == Transactions
#
# The entire callback chain of a +save+, <tt>save!</tt>, or +destroy+ call runs
# within a transaction. That includes <tt>after_*</tt> hooks. If everything
# goes fine a COMMIT is executed once the chain has been completed.
#
# If a <tt>before_*</tt> callback cancels the action a ROLLBACK is issued. You
# can also trigger a ROLLBACK raising an exception in any of the callbacks,
# including <tt>after_*</tt> 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 <tt>_*_callbacks</tt> method on an object. ActiveModel Callbacks support
# <tt>:before</tt>, <tt>:after</tt> and <tt>:around</tt> as values for the <tt>kind</tt> property. The <tt>kind</tt> 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 <tt>rest_when_dead</tt> use the <tt>filter</tt> 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_destroy_callbacks { super }
end
def touch(*) #:nodoc:
_run_touch_callbacks { super }
end
private
def create_or_update(*) #:nodoc:
_run_save_callbacks { super }
end
def _create_record #:nodoc:
_run_create_callbacks { super }
end
def _update_record(*) #:nodoc:
_run_update_callbacks { super }
end
end
end
