*********
Callbacks
*********

.. default-domain:: mongodb

.. contents:: On this page
   :local:
   :backlinks: none
   :depth: 2
   :class: singlecol

Mongoid implements many of the `ActiveRecord callbacks
<https://guides.rubyonrails.org/active_record_callbacks.html>`_.


Document Callbacks
==================

Mongoid supports the following callbacks for :doc:`documents </tutorials/documents>`:

-  ``after_initialize``
-  ``after_build``
-  ``before_validation``
-  ``after_validation``
-  ``before_create``
-  ``around_create``
-  ``after_create``
-  ``after_find``
-  ``before_update``
-  ``around_update``
-  ``after_update``
-  ``before_upsert``
-  ``around_upsert``
-  ``after_upsert``
-  ``before_save``
-  ``around_save``
-  ``after_save``
-  ``before_destroy``
-  ``around_destroy``
-  ``after_destroy``

Callbacks are available on any document, whether it is embedded within
another document or not. Note that to be efficient, Mongoid only invokes
the callback on the document that the persistence action was executed on.
This enables Mongoid to support large hierarchies and to handle optimized
atomic updates efficiently (without invoking callbacks throughout the document
hierarchy).

Note that using callbacks for domain logic is a bad design practice, and can
lead to unexpected errors that are hard to debug when callbacks in
the chain halt execution. It is our recommendation to only use them
for cross-cutting concerns, like queueing up background jobs.

.. code-block:: ruby

   class Article
     include Mongoid::Document
     field :name, type: String
     field :body, type: String
     field :slug, type: String
   
     before_create :send_message
   
     after_save do |document|
       # Handle callback here.
     end
   
     protected
     def send_message
       # Message sending code here.
     end
   end

Callbacks are coming from Active Support, so you can use the new
syntax as well:

.. code-block:: ruby

   class Article
     include Mongoid::Document
     field :name, type: String
  
     set_callback(:create, :before) do |document|
       # Message sending code here.
     end
   end


Association Callbacks
=====================

Mongoid has a set of callbacks that are specific to associations - these are:

- ``after_add``
- ``after_remove``
- ``before_add``
- ``before_remove``

Each time a document is added or removed from any of the following
associations, the respective callbacks are invoked: ``embeds_many``,
``has_many`` and ``has_and_belongs_to_many``.

Association callbacks are specified as options on the respective association.
The document added/removed will be passed as the parameter to the specified
callback. Example:

.. code-block:: ruby

   class Person
     include Mongoid::Document
   
     has_many :posts, after_add: :send_email_to_subscribers
   end
   
   def send_email_to_subscribers(post)
     Notifications.new_post(post).deliver
   end