title: Create slug with relationships
body: <p>I would like to create a slug with the combinations of relations of external tables. I would like to take 2 values ​​from a table and another value from un'atra table. I tried to do this but I do not understand how to add two values ​​and another table .. and it is required to have the fields (in my case empty)?</p>
  
  <pre><code> /**
   * @ORM\OneToOne(targetEntity="Acme\UserBundle\Entity\User", inversedBy="profile")
   * @ORM\JoinColumn(name="user_id", referencedColumnName="id")
   * */
  private $utente;
  
  /**
   * @Gedmo\Slug(handlers={
   *      @Gedmo\SlugHandler(class="Gedmo\Sluggable\Handler\RelativeSlugHandler", options={
   *          @Gedmo\SlugHandlerOption(name="relationField", value="utente"),
   *          @Gedmo\SlugHandlerOption(name="relationSlugField", value="nickname"),
   *          @Gedmo\SlugHandlerOption(name="separator", value="-")
   *      })
   * }, separator="-", updatable=true, fields={"empty"})
   * @ORM\Column(length=64, unique=true)
   */
  private $slug;
  </code></pre>
  
  <p>I would also email as well as username field,and add another relationField</p>