protected function findOrphanedHashes(): array
    {
        $queryBuilder = $this->entityManager->createQueryBuilder();
        $queryBuilder->select('if.hash AS hash')
                     ->from(IconFile::class, 'if')
                     ->leftJoin('if.icons', 'i')
                     ->andWhere('i.id IS NULL');

        $result = [];
        foreach ($queryBuilder->getQuery()->getResult() as $data) {
            $result[] = $data['hash'];
        }
        return $result;
    }