title: How to filter a model by belongsTo relationship property in EmberJS
body: <p>I'm trying to filter a findAll query to get only the elements without relationship.</p>
  
  <p>My model is created by two classes:</p>
  
  <h3>Element</h3>
  
  <pre><code>export default Model.extend({
      position: attr('number'),
      name: attr('string'),
      shared: attr('boolean', { defaultValue: false }),
      stack: belongsTo('stack')
  });
  </code></pre>
  
  <h3> Stack</h3>
  
  <pre><code>export default Model.extend({
    position: attr('number'),
    name: attr('string'),
    shared: attr('boolean', { defaultValue: false }),
    element: hasMany('element')
  });
  </code></pre>
  
  <p>I have tried to filter by property stack <code>filterBy('stack', null)</code> and <code>filterBy('stack.content', null)</code> but they didn't run.</p>
  
  <p>Any idea and help of how to filter by relationships?</p>