Observers vs. Callbacks

Question

i thought about using observers or callbacks. What and when you should use an observer?

F.e. you could do following:

# User-model class User << AR   after_create :send_greeting!    def send_greeting!     UserNotifier.deliver_greeting_message(self)   end  end  #observer class UserNotifier << AR   def greeting_message(user)   ...   end end 

or you could create an observer and let it watch when users becomes created...

What dou you recommened?

Answer 1

One really important distinction to keep in mind, which is related to Milan Novota's answer, is that callbacks on an ActiveRecord have the ability to cancel the action being called and all subsequent callbacks, where as observers do not.

class Model < ActiveRecord::Base   before_update :disallow_bob    def disallow_bob   return false if model.name == "bob"   end end  class ModelObserver < ActiveRecord::Observer   def before_update(model)     return false if model.name == "mary"   end end  m = Model.create(:name => "whatever")  m.update_attributes(:name => "bob") => false -- name will still be "whatever" in database  m.update_attributes(:name => "mary") => true -- name will be "mary" in database 

Observers may only observe, they may not intervene.