FOSUserBundle Entity constuct

I have entity User which extends FOSUserBundle.

class User extends BaseUser{...    protected $ref_id;    /**     * @ORM\Column(type="float", length="11")...

ref_id comes from cookies variable ref_id. How can i pass this value to entity User each times new user is registering. Do i have to override registerAction or create constructor on entity User and somehow pass ref_id there ?

Additionaly, i need to validate (check from database) ref_id every time new user is registering. How can i do that ?

    class User extends BaseUser    {    ...       public function __construct()       {           $em = $this->getDoctrine()->getEntityManager();

This example doesn't work.