Dynamic relationship and the SonataAdminBundle - doctrine-orm

Question

I use the SonataAdminBundle for the first time in my life and I have some
problems with it.
At first, I had a PageBundle which has a Page and a Author entity. Then I
started using SonataAdminBundle and used the sonata_type_model to nicely
display the Authors of a Page:
// ...
protected function configureFormFields(FormMapper $mapper)
{
$mapper
->add('title')
->add('slug', null, array('required' => false))
->add('published', null, array(
'label' => 'publish',
'required' => false,
))
->add('author', 'sonata_type_model')
->add('content')
;
}
But then I discovered the SonataUserBundle. I started using it and when I
finally get it working I thought it would be nice to make use of this User
entity, instead of the Author entity in the PageBundle. To make this possible,
I used the technique where the documentation talks about in
"How to Define Relationships with Abstract Classes and Interfaces".
This worked, but not with the SonataAdminBundle. It seems like the
sonata_type_model doesn't work with the ResolveTargetEntityListener and I
can't make it working.
The relevant Page entity code in the PageBundle:
namespace Wj\PageBundle\Entity;
use Doctrine\ORM\Mapping as ORM;
/**
* #ORM\Table
* #ORM\Entity
* #ORM\HasLifeCycleCallbacks
*/
class Page
{
/**
* #var integer $authorId
*
* #ORM\ManyToOne(targetEntity="Wj\PageBundle\Model\AuthorInterface")
*/
private $author;
}
The Wj\PageBundle\Model\AuthorInterface:
namespace Wj\PageBundle\Model;
interface AuthorInterface
{
public function getId();
}
And the User entity in the UserBundle:
namespace Wj\UserBundle\Entity;
use Sonata\UserBundle\Entity\BaseUser;
use Wj\PageBundle\Model\AuthorInterface;
use Doctrine\ORM\Mapping as ORM;
/**
* #ORM\Entity
* #ORM\Table(name="fos_user")
*/
class User extends BaseUser implements AuthorInterface
{
/**
* #ORM\Id
* #ORM\Column(type="integer")
* #ORM\GeneratedValue(strategy="AUTO")
*/
protected $id;
public function __toString()
{
return $this->getFirstName().' '.$this->getLastName();
}
}
The FormMapper configuration is the same as I posted above.
How can I use the ResolveTargetEntityListener technique in combination with
the SonataAdminBundle and his sonata_type_model?