public function findByUsernames(array $usernames)
    {
        if (count($usernames) > 0) {
            $dql = '
                SELECT u FROM Claroline\CoreBundle\Entity\User u
                WHERE u.isRemoved = false
                AND u.username IN (:usernames)
            ';

            $query = $this->_em->createQuery($dql);
            $query->setParameter('usernames', $usernames);
            $result = $query->getResult();
        } else {
            $result = [];
        }

        return $result;
    }