title: changed? method not work inside model
body: <p>I have the following model:</p>
  
  <pre><code>class TicketStatus &lt; ActiveRecord::Base
    after_update :set_previous_staff
    attr_accessible :status_id, :ticket_id , :staff_id, :advisor_id, :previous_advisor_id, :previous_staff_id
    belongs_to :status
    belongs_to :ticket
    belongs_to :staff, class_name: 'Staff', foreign_key: 'staff_id'
    belongs_to :previous_staff , class_name: 'Staff', foreign_key: 'previous_staff_id'
    belongs_to :advisor, class_name: 'Advisor', foreign_key: 'advisor_id'
  
    def set_previous_staff
      self.previous_staff_id = self.staff_id_was if self.staff_id_changed?
    end
  
  end
  </code></pre>
  
  <p><code>set_previous_staff</code> method is not working with the callback however i tried <code>changed?</code> in console and it should work , what is missing here ? i am using rails 3.2.14</p>
  
  <p>what i expected: I have an assigned employee to a ticket , i need when that assigned employee changed, his id should be added to <code>previous_staff_id</code> , so this will let me know who is the current employee and who was before him.</p>