title: Custom choices list of sonata_type_model field with Sonata Admin
body: <p>I am using Sonata Admin and I have a field of categories and I need to show them in order like a tree in select:</p>
  
  <pre><code>&lt;select&gt;
      &lt;option&gt;Category father-1&lt;/option&gt;
      &lt;option&gt;--Category child-1-1&lt;/option&gt;
      &lt;option&gt;--Category child-1-2&lt;/option&gt;
      &lt;option&gt;--Category child-1-3&lt;/option&gt;
      &lt;option&gt;----Category child-1-3-1&lt;/option&gt;
      &lt;option&gt;----Category child-1-3-2&lt;/option&gt;
      &lt;option&gt;--Category child-1-4&lt;/option&gt;
      &lt;option&gt;--...&lt;/option&gt;
      &lt;option&gt;Category father-2&lt;/option&gt;
  &lt;/select&gt;
  </code></pre>
  
  <p>It's possible? I have tried it including in 'choice_list' an array generate in getTreeCatsArray method:</p>
  
  <pre><code>protected function configureFormFields(FormMapper $formMapper)
  {
      $tree_cat_array = $this-&gt;em-&gt;getRepository('MyBundle:Category')-&gt;getTreeCatsArray();
  
      $formMapper
          -&gt;add('category', 'sonata_type_model', array(
                  'empty_value' =&gt; '', 
                  'choice_list' =&gt; $tree_cat_array)); 
  }
  </code></pre>
  
  <p>This shows the error:</p>
  
  <pre><code>The option "choice_list" with value "Array" is expected to be of type "null", "Symfony\Component\Form\Extension\Core\ChoiceList\ChoiceListInterface"
  </code></pre>
  
  <p>I am not sure if I must use field type 'sonata_type_model' or 'choice'</p>