public function getClassnames(array $contentUids)
    {
        $contentUids = array_map(function ($uid) {
            return '"'.$uid.'"';
        }, array_filter($contentUids));

        if (0 === count($contentUids)) {
            return [];
        }

        $qb = $this->_em->getConnection()->createQueryBuilder();

        return $qb
            ->select('DISTINCT c.classname')
            ->from('content', 'c')
            ->where($qb->expr()->in('c.uid', $contentUids))
            ->execute()
            ->fetchAll(\PDO::FETCH_COLUMN)
        ;
    }