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.

class ContactObserver < ActiveRecord::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 ::observe class method which takes either the concrete class (Product) or a symbol for that class (:product):

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.

Storing Observers in Rails

If you're using Active Record 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.active_record.observersconfiguration setting in your config/environment.rbfile.

config.active_record.observers = :comment_observer, :signup_observer

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

Loading

Observers register themselves in the model class 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.

Up to (and including) Rails 2.0.2 observers were instantiated between plugins and application initializers. Now observers are loaded after application initializers, so 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.instancebefore. Observers are singletons and that call instantiates and registers them.

Methods
A
N
O
Included Modules
Class Public methods
new()

Start observing the declared classes and their subclasses.

# File activerecord/lib/active_record/observer.rb, line 165
def initialize
  Set.new(observed_classes + observed_subclasses).each { |klass| add_observer! klass }
end
observe(*models)

Attaches the observer to the supplied model classes.

# File activerecord/lib/active_record/observer.rb, line 147
def observe(*models)
  models.flatten!
  models.collect! { |model| model.is_a?(Symbol) ? model.to_s.camelize.constantize : model }
  define_method(:observed_classes) { Set.new(models) }
end
observed_class()

The class observed by default is inferred from the observer's class name:

assert_equal Person, PersonObserver.observed_class
# File activerecord/lib/active_record/observer.rb, line 155
def observed_class
  if observed_class_name = name[/(.*)Observer/, 1]
    observed_class_name.constantize
  else
    nil
  end
end
Instance Protected methods
add_observer!(klass)
# File activerecord/lib/active_record/observer.rb, line 190
def add_observer!(klass)
  klass.add_observer(self)
  if respond_to?(:after_find) && !klass.method_defined?(:after_find)
    klass.class_eval 'def after_find() end'
  end
end
observed_classes()
# File activerecord/lib/active_record/observer.rb, line 182
def observed_classes
  Set.new([self.class.observed_class].compact.flatten)
end
observed_subclasses()
# File activerecord/lib/active_record/observer.rb, line 186
def observed_subclasses
  observed_classes.sum([]) { |klass| klass.send(:subclasses) }
end