title: Computed property in Emberjs Model
body: <p>Suppose we have a model like this:</p>
  
  <pre><code>App.Somemodel = DS.Model.extend({
      A:DS.attr('number'),
      B:DS.attr('number'),
      CP: function(){return this.get('A') + this.get('B');}.property('A','B')
  })
  </code></pre>
  
  <p>My question is this :When is CP going to be re calculated ?
  - Right after A changed (before record.save() )
  - After the record is saved ?
  - At the moment when this record is retrieved from store again ?</p>
  
  <p>My second question is : Is the behavior because of the computed property itself ?</p>
  
  <p>I am now struggling with some issues:  when updating 1000+ records at the same time ,The efficiency get terrible .</p>
  
  <p>I was trying to understand :Is it a good idea to define computed-property in the model ?</p>