How can an observer find out the before and after values of the observed property in Ember.js?

Question

Please see the solution to another question provided by ud3323: http://jsfiddle.net/ud3323/ykL69/. This solution highlights the changed value using the red color. I have an

Answer 1

I used a computed property to route to a model's property and store the last value upon its set.

registrationGroup: Em.computed('regionModel.registrationGroup', 
get:->
  @.get('regionModel.registrationGroup')
set:(key, value, oldValue)->
  @.set('lastRegistrationGroup', oldValue)
  @.set('regionModel.registrationGroup', value)
)