Active Model Observers

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. Example: 

  class CommentObserver < ActiveModel::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 < ActiveModel::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 < ActiveModel::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 < ActiveModel::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.

If you’re using an Observer in a Rails application with Active Record, be sure to read about the necessary configuration in the documentation for ActiveRecord::Observer.

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

Start observing the declared classes and their subclasses.

Source: show | on GitHub 

     # File activemodel/lib/active_model/observing.rb, line 218
218:     def initialize
219:       observed_classes.each { |klass| add_observer!(klass) }
220:     end
observe(*models)

Attaches the observer to the supplied model classes.

Source: show | on GitHub 

     # File activemodel/lib/active_model/observing.rb, line 187
187:       def observe(*models)
188:         models.flatten!
189:         models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
190:         redefine_method(:observed_classes) { models }
191:       end
observed_class()

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

  assert_equal Person, PersonObserver.observed_class

Source: show | on GitHub 

     # File activemodel/lib/active_model/observing.rb, line 208
208:       def observed_class
209:         if observed_class_name = name[/(.*)Observer/, 1]
210:           observed_class_name.constantize
211:         else
212:           nil
213:         end
214:       end
observed_classes()

Returns an array of Classes to observe.

You can override this instead of using the observe helper. 

  class AuditObserver < ActiveModel::Observer
    def self.observed_classes
      [Account, Balance]
    end
  end

Source: show | on GitHub 

     # File activemodel/lib/active_model/observing.rb, line 202
202:       def observed_classes
203:         Array.wrap(observed_class)
204:       end
Instance Protected methods
disabled_for?(object)

Source: show | on GitHub 

     # File activemodel/lib/active_model/observing.rb, line 246
246:       def disabled_for?(object)
247:         klass = object.class
248:         return false unless klass.respond_to?(:observers)
249:         klass.observers.disabled_for?(self)
250:       end