Class: Mongoid::Observer

Inherits:
ActiveModel::Observer
  • Object
show all
Defined in:
lib/mongoid/observer.rb

Overview

Observer classes respond to life cycle callbacks to implement trigger-like behavior outside the original class. This is a great way to reduce the clutter that normally comes when the model class is burdened with functionality that doesn’t pertain to the core responsibility of the class. Mongoid’s observers work similar to ActiveRecord’s. Example:

class CommentObserver < Mongoid::Observer
  def after_save(comment)
    Notifications.comment(
      "[email protected]", "New comment was posted", comment
    ).deliver
  end
end

This Observer sends an email when a Comment#save is finished.

class ContactObserver < Mongoid::Observer
  def after_create(contact)
    contact.logger.info('New contact added!')
  end

  def after_destroy(contact)
    contact.logger.warn("Contact with an id of #{contact.id} was destroyed!")
  end
end

This Observer uses logger to log when specific callbacks are triggered.

Observing a class that can’t be inferred

Observers will by default be mapped to the class with which they share a name. So CommentObserver will be tied to observing Comment, ProductManagerObserver to ProductManager, and so on. If you want to name your observer differently than the class you’re interested in observing, you can use the Observer.observe class method which takes either the concrete class (Product) or a symbol for that class (:product):

class AuditObserver < Mongoid::Observer
  observe :account

  def after_update()
    AuditTrail.new(, "UPDATED")
  end
end

If the audit observer needs to watch more than one kind of object, this can be specified with multiple arguments:

class AuditObserver < Mongoid::Observer
  observe :account, :balance

  def after_update(record)
    AuditTrail.new(record, "UPDATED")
  end
end

The AuditObserver will now act on both updates to Account and Balance by treating them both as records.

Available callback methods

  • after_initialize

  • before_validation

  • after_validation

  • before_create

  • around_create

  • after_create

  • before_update

  • around_update

  • after_update

  • before_upsert

  • around_upsert

  • after_upsert

  • before_save

  • around_save

  • after_save

  • before_destroy

  • around_destroy

  • after_destroy

Storing Observers in Rails

If you’re using Mongoid within Rails, observer classes are usually stored in app/models with the naming convention of app/models/audit_observer.rb.

Configuration

In order to activate an observer, list it in the config.mongoid.observers configuration setting in your config/application.rb file.

config.mongoid.observers = :comment_observer, :signup_observer

Observers will not be invoked unless you define them in your application configuration.

Loading

Observers register themselves with the model class that they observe, since it is the class that notifies them of events when they occur. As a side-effect, when an observer is loaded, its corresponding model class is loaded.

Observers are loaded after the application initializers, so that observed models can make use of extensions. If by any chance you are using observed models in the initialization, you can still load their observers by calling ModelObserver.instance before. Observers are singletons and that call instantiates and registers them.

Class Method Summary collapse

Class Method Details

.observe(*models) ⇒ Object

Attaches the observer to the specified classes.

Examples:

Attach the BandObserver to the class Artist.

class BandObserver < Mongoid::Observer
  observe :artist
end

Parameters:

  • models (Array<Symbol>) —

    The names of the models.



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# File 'lib/mongoid/observer.rb', line 177

def observe(*models)
  models.flatten!
  models.collect! do |model|
    model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model
  end
  singleton_class.redefine_method(:observed_classes) { models }
end