callbacks.rb

require 'active_support/core_ext/array/wrap'

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>.
  #
  # 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.
  #
  # 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 <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" or 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 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 <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.
  #
  # == 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
    ]

    included do
      extend ActiveModel::Callbacks
      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