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.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 < 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.

     # File lib/active_model/observing.rb, line 190
190:     def initialize
191:       observed_classes.each { |klass| add_observer!(klass) }
192:     end

     # File lib/active_model/observing.rb, line 158
158:       def observe(*models)
159:         models.flatten!
160:         models.collect! { |model| model.respond_to?(:to_sym) ? model.to_s.camelize.constantize : model }
161:         remove_possible_method(:observed_classes)
162:         define_method(:observed_classes) { models }
163:       end

  assert_equal Person, PersonObserver.observed_class

     # File lib/active_model/observing.rb, line 180
180:       def observed_class
181:         if observed_class_name = name[/(.*)Observer/, 1]
182:           observed_class_name.constantize
183:         else
184:           nil
185:         end
186:       end

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

     # File lib/active_model/observing.rb, line 174
174:       def observed_classes
175:         Array.wrap(observed_class)
176:       end