1 The Object Life Cycle
During the normal operation of a Rails application, objects may be created, updated, and destroyed. Active Record provides hooks into this object life cycle so that you can control your application and its data.
Callbacks allow you to trigger logic before or after an alteration of an object's state.
2 Callbacks Overview
Callbacks are methods that get called at certain moments of an object's life cycle. With callbacks it is possible to write code that will run whenever an Active Record object is created, saved, updated, deleted, validated, or loaded from the database.
2.1 Callback Registration
In order to use the available callbacks, you need to register them. You can implement the callbacks as ordinary methods and use a macro-style class method to register them as callbacks:
class User < ActiveRecord::Base validates :login, :email, presence: true before_validation :ensure_login_has_a_value protected def ensure_login_has_a_value if login.nil? self.login = email unless email.blank? end end end
The macro-style class methods can also receive a block. Consider using this style if the code inside your block is so short that it fits in a single line:
class User < ActiveRecord::Base validates :login, :email, presence: true before_create do self.name = login.capitalize if name.blank? end end
Callbacks can also be registered to only fire on certain life cycle events:
class User < ActiveRecord::Base before_validation :normalize_name, on: :create # :on takes an array as well after_validation :set_location, on: [ :create, :update ] protected def normalize_name self.name = self.name.downcase.titleize end def set_location self.location = LocationService.query(self) end end
It is considered good practice to declare callback methods as protected or private. If left public, they can be called from outside of the model and violate the principle of object encapsulation.
3 Available Callbacks
Here is a list with all the available Active Record callbacks, listed in the same order in which they will get called during the respective operations:
3.1 Creating an Object
before_validation
after_validation
before_save
around_save
before_create
around_create
after_create
after_save
after_commit/after_rollback
3.2 Updating an Object
before_validation
after_validation
before_save
around_save
before_update
around_update
after_update
after_save
after_commit/after_rollback
3.3 Destroying an Object
before_destroy
around_destroy
after_destroy
after_commit/after_rollback
after_save
runs both on create and update, but always after the more specific callbacks after_create
and after_update
, no matter the order in which the macro calls were executed.
3.4 after_initialize
and after_find
The after_initialize
callback will be called whenever an Active Record object is instantiated, either by directly using new
or when a record is loaded from the database. It can be useful to avoid the need to directly override your Active Record initialize
method.
The after_find
callback will be called whenever Active Record loads a record from the database. after_find
is called before after_initialize
if both are defined.
The after_initialize
and after_find
callbacks have no before_*
counterparts, but they can be registered just like the other Active Record callbacks.
class User < ActiveRecord::Base after_initialize do |user| puts "You have initialized an object!" end after_find do |user| puts "You have found an object!" end end >> User.new You have initialized an object! => #<User id: nil> >> User.first You have found an object! You have initialized an object! => #<User id: 1>
3.5 after_touch
The after_touch
callback will be called whenever an Active Record object is touched.
class User < ActiveRecord::Base after_touch do |user| puts "You have touched an object" end end >> u = User.create(name: 'Kuldeep') => #<User id: 1, name: "Kuldeep", created_at: "2013-11-25 12:17:49", updated_at: "2013-11-25 12:17:49"> >> u.touch You have touched an object => true
It can be used along with belongs_to
:
class Employee < ActiveRecord::Base belongs_to :company, touch: true after_touch do puts 'An Employee was touched' end end class Company < ActiveRecord::Base has_many :employees after_touch :log_when_employees_or_company_touched private def log_when_employees_or_company_touched puts 'Employee/Company was touched' end end >> @employee = Employee.last => #<Employee id: 1, company_id: 1, created_at: "2013-11-25 17:04:22", updated_at: "2013-11-25 17:05:05"> # triggers @employee.company.touch >> @employee.touch Employee/Company was touched An Employee was touched => true
4 Running Callbacks
The following methods trigger callbacks:
create
create!
decrement!
destroy
destroy!
destroy_all
increment!
save
save!
save(validate: false)
toggle!
update_attribute
update
update!
valid?
Additionally, the after_find
callback is triggered by the following finder methods:
all
first
find
find_by
find_by_*
find_by_*!
find_by_sql
last
The after_initialize
callback is triggered every time a new object of the class is initialized.
The find_by_*
and find_by_*!
methods are dynamic finders generated automatically for every attribute. Learn more about them at the Dynamic finders section
5 Skipping Callbacks
Just as with validations, it is also possible to skip callbacks by using the following methods:
decrement
decrement_counter
delete
delete_all
increment
increment_counter
toggle
touch
update_column
update_columns
update_all
update_counters
These methods should be used with caution, however, because important business rules and application logic may be kept in callbacks. Bypassing them without understanding the potential implications may lead to invalid data.
6 Halting Execution
As you start registering new callbacks for your models, they will be queued for execution. This queue will include all your model's validations, the registered callbacks, and the database operation to be executed.
The whole callback chain is wrapped in a transaction. If any before callback method returns exactly false
or raises an exception, the execution chain gets halted and a ROLLBACK is issued; after callbacks can only accomplish that by raising an exception.
Any exception that is not ActiveRecord::Rollback
will be re-raised by Rails after the callback chain is halted. Raising an exception other than ActiveRecord::Rollback
may break code that does not expect methods like save
and update_attributes
(which normally try to return true
or false
) to raise an exception.
7 Relational Callbacks
Callbacks work through model relationships, and can even be defined by them. Suppose an example where a user has many articles. A user's articles should be destroyed if the user is destroyed. Let's add an after_destroy
callback to the User
model by way of its relationship to the Article
model:
class User < ActiveRecord::Base has_many :articles, dependent: :destroy end class Article < ActiveRecord::Base after_destroy :log_destroy_action def log_destroy_action puts 'Article destroyed' end end >> user = User.first => #<User id: 1> >> user.articles.create! => #<Article id: 1, user_id: 1> >> user.destroy Article destroyed => #<User id: 1>
8 Conditional Callbacks
As with validations, we can also make the calling of a callback method conditional on the satisfaction of a given predicate. We can do this using the :if
and :unless
options, which can take a symbol, a string, a Proc
or an Array
. You may use the :if
option when you want to specify under which conditions the callback should be called. If you want to specify the conditions under which the callback should not be called, then you may use the :unless
option.
8.1 Using :if
and :unless
with a Symbol
You can associate the :if
and :unless
options with a symbol corresponding to the name of a predicate method that will get called right before the callback. When using the :if
option, the callback won't be executed if the predicate method returns false; when using the :unless
option, the callback won't be executed if the predicate method returns true. This is the most common option. Using this form of registration it is also possible to register several different predicates that should be called to check if the callback should be executed.
class Order < ActiveRecord::Base before_save :normalize_card_number, if: :paid_with_card? end
8.2 Using :if
and :unless
with a String
You can also use a string that will be evaluated using eval
and hence needs to contain valid Ruby code. You should use this option only when the string represents a really short condition:
class Order < ActiveRecord::Base before_save :normalize_card_number, if: "paid_with_card?" end
8.3 Using :if
and :unless
with a Proc
Finally, it is possible to associate :if
and :unless
with a Proc
object. This option is best suited when writing short validation methods, usually one-liners:
class Order < ActiveRecord::Base before_save :normalize_card_number, if: Proc.new { |order| order.paid_with_card? } end
8.4 Multiple Conditions for Callbacks
When writing conditional callbacks, it is possible to mix both :if
and :unless
in the same callback declaration:
class Comment < ActiveRecord::Base after_create :send_email_to_author, if: :author_wants_emails?, unless: Proc.new { |comment| comment.article.ignore_comments? } end
9 Callback Classes
Sometimes the callback methods that you'll write will be useful enough to be reused by other models. Active Record makes it possible to create classes that encapsulate the callback methods, so it becomes very easy to reuse them.
Here's an example where we create a class with an after_destroy
callback for a PictureFile
model:
class PictureFileCallbacks def after_destroy(picture_file) if File.exist?(picture_file.filepath) File.delete(picture_file.filepath) end end end
When declared inside a class, as above, the callback methods will receive the model object as a parameter. We can now use the callback class in the model:
class PictureFile < ActiveRecord::Base after_destroy PictureFileCallbacks.new end
Note that we needed to instantiate a new PictureFileCallbacks
object, since we declared our callback as an instance method. This is particularly useful if the callbacks make use of the state of the instantiated object. Often, however, it will make more sense to declare the callbacks as class methods:
class PictureFileCallbacks def self.after_destroy(picture_file) if File.exist?(picture_file.filepath) File.delete(picture_file.filepath) end end end
If the callback method is declared this way, it won't be necessary to instantiate a PictureFileCallbacks
object.
class PictureFile < ActiveRecord::Base after_destroy PictureFileCallbacks end
You can declare as many callbacks as you want inside your callback classes.
10 Transaction Callbacks
There are two additional callbacks that are triggered by the completion of a database transaction: after_commit
and after_rollback
. These callbacks are very similar to the after_save
callback except that they don't execute until after database changes have either been committed or rolled back. They are most useful when your active record models need to interact with external systems which are not part of the database transaction.
Consider, for example, the previous example where the PictureFile
model needs to delete a file after the corresponding record is destroyed. If anything raises an exception after the after_destroy
callback is called and the transaction rolls back, the file will have been deleted and the model will be left in an inconsistent state. For example, suppose that picture_file_2
in the code below is not valid and the save!
method raises an error.
PictureFile.transaction do picture_file_1.destroy picture_file_2.save! end
By using the after_commit
callback we can account for this case.
class PictureFile < ActiveRecord::Base after_commit :delete_picture_file_from_disk, on: [:destroy] def delete_picture_file_from_disk if File.exist?(filepath) File.delete(filepath) end end end
the :on
option specifies when a callback will be fired. If you
don't supply the :on
option the callback will fire for every action.
The after_commit
and after_rollback
callbacks are guaranteed to be called for all models created, updated, or destroyed within a transaction block. If any exceptions are raised within one of these callbacks, they will be ignored so that they don't interfere with the other callbacks. As such, if your callback code could raise an exception, you'll need to rescue it and handle it appropriately within the callback.
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