public function findAdminHierarchiesByParent(WorkspaceTag $workspaceTag)
    {
        $dql = '
            SELECT h
            FROM Claroline\CoreBundle\Entity\Workspace\WorkspaceTagHierarchy h
            WHERE h.user IS NULL
            AND h.parent = :workspaceTag
        ';
        $query = $this->_em->createQuery($dql);
        $query->setParameter('workspaceTag', $workspaceTag);

        return $query->getResult();
    }