Class Mongoid::Observer
In: lib/mongoid/observer.rb
Parent: ActiveModel::Observer

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(
        "admin@do.com", "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(account)
      AuditTrail.new(account, "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_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.

Methods

Public Class methods

Instantiate the new observer. Will add all child observers as well.

@example Instantiate the observer.

  Mongoid::Observer.new

@since 2.0.0.rc.8

Protected Instance methods

Adds the specified observer to the class.

@example Add the observer.

  observer.add_observer!(Document)

@param [ Class ] klass The child observer to add.

@since 2.0.0.rc.8

Defines all the callbacks for each observer of the model.

@example Define all the callbacks.

  observer.define_callbacks(Document)

@param [ Class ] klass The model to define them on.

@since 2.0.0.rc.8

Get all the child observers.

@example Get the children.

  observer.observed_descendants

@return [ Array<Class> ] The children.

@since 2.0.0.rc.8

[Validate]