public function deletePreference(User $user, $key)
{
$dql = "DELETE FROM PartKeepr\\AuthBundle\\Entity\\UserPreference up WHERE up.user = :user AND ";
$dql .= 'up.preferenceKey = :key';
$query = $this->entityManager->createQuery($dql);
$query->setParameter('user', $user);
$query->setParameter('key', $key);
$query->execute();
}