title: Create a Symfony form drop-down list with two properties
body: <p>I'm trying to make a Symfony form with a drop-down list which use two properties.</p>
  
  <p>Let me explain :
  I have a SparePart entity with two properties :</p>
  
  <pre><code>/**
  * @ORM\ManyToOne(targetEntity="CM\PlatformBundle\Entity\Supplier")
  */
  private $supplier;
  
  /**
  * @ORM\ManyToOne(targetEntity="CM\PlatformBundle\Entity\SupplierGroup")
  */
  private $supplierGroup;
  </code></pre>
  
  <p>I want to let the user chose between either one direct supplier or a group of supplier, like this :</p>
  
  <ol>
  <li>----DIRECT SUPPLIERS----
  
  <ul>
  <li>Michelin</li>
  <li>Continental</li>
  </ul></li>
  <li>----GROUP OF SUPPLIERS----
  
  <ul>
  <li>European suppliers</li>
  <li>American suppliers</li>
  </ul></li>
  </ol>
  
  <p>I don't know how to handle this kind of situation in the form builder.</p>