title: Ember Model Object hasMany does not return array as I would expect
body: <p>I am trying to use Ember Data's hasMany field to return me an array and get the count of the items in the array as a computed property. However, when I do the following, it returns me an object (which appears to a promise because of the {async: true}?) instead of the array I expected.</p>
  
  <pre><code>App.Shift = DS.Model.extend({
    name: DS.attr('string'),
    people: DS.hasMany('person', {async: true});
    number_of_people: (function(){
      this.get('people').length
    }).property('people')
  )};
  
  App.Person = DS.Model.extend({
    first_name: DS.attr('string'),
    last_name: DS.attr('string')
  });
  </code></pre>
  
  <p><strong>Update:</strong> I would like to return the length of people. I tried this but when I access the property I get the promise object returned still instead of the value of completed promise in the then. How would I get the value of the evaluated promised to be returned?</p>
  
  <pre><code>number_of_people: (function(){
    return this.get('people').then(function(people){
      return people.get('length');
    });
  })
  </code></pre>