title: Symfony2 ondelete set Null
body: <p>I'm new with symfony2, and I started a project of blog. This project has two entities, one of posts (<code>Noticia</code>) and one with comments (<code>Comentario</code>).
  I need to delete a post but I can't because of the relationship with comments.</p>
  
  <p>Here is my attribute:</p>
  
  <pre><code>/**
   * @ORM\ManyToOne(targetEntity="Noticia", inversedBy="comentarios", onDelete="SET NULL")
   * @ORM\JoinColumn(name="noticia_id", referencedColumnName="id")
   */
  private $noticia;
  </code></pre>
  
  <hr>
  
  <pre><code>/**
   * @ORM\OneToMany(targetEntity="Comentario", mappedBy="noticia")
   */
  private $comentarios = array();
  </code></pre>
  
  <p>I tried to put <code>onDelete="SET NULL"</code> but, when I updated my database with doctrine: <code>php app/console doctrine:schema:update --force</code> I got this error:</p>
  
  <pre><code>The annotation @ORM\ManyToOne declared on property Noticia 
  Bundle\Entity\Comentario\::$noticia does not have a property named "onDelete".
  Avaible properties: targetEntity, cascade, fetch, inversedBy
  </code></pre>