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 ActiveRecord::Base#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:

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.observers configuration setting in your config/environment.rb file.

config.active_record.observers = :comment_observer, :signup_observer

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

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

Start observing the declared classes and their subclasses.

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 148
def initialize
  Set.new(observed_classes + observed_subclasses).each { |klass| add_observer! klass }
end
observe(*models)

Attaches the observer to the supplied model classes.

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 136
def observe(*models)
  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

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 142
def observed_class
  name.scan(%r(.*)Observer/)[0][0].constantize
end
Instance Protected methods
add_observer!(klass)

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 173
def add_observer!(klass)
  klass.add_observer(self)
  klass.class_eval 'def after_find() end' unless klass.respond_to?(:after_find)
end
observed_classes()

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 165
def observed_classes
  Set.new([self.class.observed_class].flatten)
end
observed_subclasses()

Source: show 

# File rails/activerecord/lib/active_record/observer.rb, line 169
def observed_subclasses
  observed_classes.sum(&:subclasses)
end