Mongoid::Observers

Mongoid Observers (removed from core in Mongoid 4.0). Because this gem doesn't exist and I need to use it very often. Therefore, I extract the code from mongoid on my own. It's basically the same code from mongoid before it's removed.

Installation

Add this line to your application's Gemfile:

gem 'mongoid-observers'

And then execute:

$ bundle

Or install it yourself as:

$ gem install mongoid-observers

Usage

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_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.

Authors

Chamnap Chhorn

Bunlong/mongoid-observers