Class: ActiveRecord::Observer

Inherits:

Object

• Object
• ActiveRecord::Observer

Includes:

Singleton

Defined in:

lib/active_record/observer.rb

Overview

Observer classes respond to lifecycle 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. Example:

class CommentObserver < ActiveRecord::Observer
  def after_save(comment)
    Notifications.deliver_comment("admin@do.com", "New comment was posted", comment)
  end
end

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

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:

class AuditObserver < ActiveRecord::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 < ActiveRecord::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

The observer can implement callback methods for each of the methods described in the Callbacks module.

Triggering Observers

In order to activate an observer, you need to call Observer.instance. In Rails, this can be done in controllers using the short-hand of for example observer :comment_observer.

Class Method Summary

• .observe(*models) ⇒ Object

  Attaches the observer to the supplied model classes.

Instance Method Summary

• #initialize ⇒ Observer constructor

  A new instance of Observer.

• #update(callback_method, object) ⇒ Object

  :nodoc:.

Constructor Details

#initialize ⇒ Observer

Returns a new instance of Observer.

# File 'lib/active_record/observer.rb', line 60

def initialize
  [ observed_class ].flatten.each do |klass| 
    klass.add_observer(self)
    klass.send(:define_method, :after_find) unless klass.respond_to?(:after_find)
  end
end

Class Method Details

.observe(*models) ⇒ Object

Attaches the observer to the supplied model classes.

# File 'lib/active_record/observer.rb', line 56

def self.observe(*models)
  define_method(:observed_class) { models }
end

Instance Method Details

#update(callback_method, object) ⇒ Object

:nodoc:

# File 'lib/active_record/observer.rb', line 67

def update(callback_method, object) #:nodoc:
  send(callback_method, object) if respond_to?(callback_method)
end