Guides.rubyonrails.org

Chapters

1. The Object Life Cycle
2. Callbacks Overview
   • Callback Registration
3. Available Callbacks
   • Creating an Object
   • Updating an Object
   • Destroying an Object
   • after_initialize and after_find
4. Running Callbacks
5. Skipping Callbacks
6. Halting Execution
7. Relational Callbacks
8. Conditional Callbacks
   • Using :if and :unless with a Symbol
   • Using :if and :unless with a String
   • Using :if and :unless with a Proc
   • Multiple Conditions for Callbacks
9. Callback Classes
10. Transaction Callbacks

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 |user|
    user.name = user.login.capitalize if user.name.blank?
  end
end

Callbacks can also be registered to only fire on certain lifecycle 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

3.2 Updating an Object

• before_validation
• after_validation
• before_save
• around_save
• before_update
• around_update
• after_update
• after_save

3.3 Destroying an Object

• before_destroy
• around_destroy
• after_destroy

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>

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.

5 Skipping Callbacks

Just as with validations, it is also possible to skip callbacks. 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.

• decrement
• decrement_counter
• delete
• delete_all
• increment
• increment_counter
• toggle
• touch
• update_column
• update_columns
• update_all
• update_counters

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.

7 Relational Callbacks

Callbacks work through model relationships, and can even be defined by them. Suppose an example where a user has many posts. A user's posts 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 Post model:

class User < ActiveRecord::Base
  has_many :posts, dependent: :destroy
end

class Post < ActiveRecord::Base
  after_destroy :log_destroy_action

  def log_destroy_action
    puts 'Post destroyed'
  end
end

>> user = User.first
=> #<User id: 1>
>> user.posts.create!
=> #<Post id: 1, user_id: 1>
>> user.destroy
Post 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.post.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.exists?(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.exists?(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 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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