title: Backbone.js collection get does not work
body: <pre><code>var sgt1 = Backbone.Collection.extend({
          model: sgt2
      });
  
  var a = new sgt1();
  a.add({attr1: 'asd'});
  </code></pre>
  
  <p>Ok, up to now it is works but:</p>
  
  <p><code>a.get(0)</code> returns undefined</p>
  
  <p>and</p>
  
  <p><code>a.at(0)</code> returns an obj</p>
  
  <p>Why does <code>get</code> return <code>undefined</code>?</p>