title: Formatting choice list options in entity form type
body: <p>i'm using Entity form type, choice based, in order to provide a choice list inside a Sonata Admin form. What i would like is to use a child property to display the collection of elements inside the choice list.  </p>
  
  <p>Does any one knows how to provide the label to be shown inside the chocie list or how to give format to the choice list array?</p>
  
  <pre><code>    $formMapper-&gt;add( 'frArea', 'entity', array(
          'class'             =&gt; 'myVendor\myBundleBundle\View\myEntity',
          //When using a child property here, choice list gets corrupted/bad formatted
          //'property'            =&gt; 'child.property',
          'property'          =&gt; 'property',
          'em'                =&gt; 'formacions',
          'query_builder'     =&gt;  function( EntityRepository $er )
                                  {
                                      return $er-&gt;createQueryBuilder( 'qb' )
                                                -&gt;add( 'select', 'm' )
                                                -&gt;add( 'from', 'myVendor\myBundleBundle\View\myEntity m' );
                                                //-&gt;leftJoin('a.idiomes', 'ai');
                                  },
          'required'          =&gt; false, 
          'label'             =&gt; ucfirst( $this-&gt;trans( 'my_label', array(), $this-&gt;translationDomain, $this-&gt;langCode ) )
      ) );  
  </code></pre>
  
  <p>When using property option in the following way <code>'property' =&gt; 'child.property'</code> i get the following error:  </p>
  
  <pre><code>Neither property "nom" nor method "getProperty()" nor method "isProperty()" exists in class "Doctrine\ORM\PersistentCollection"
  </code></pre>