public function findUnfinishedPapers(Exercise $exercise, User $user)
    {
        return $this->createQueryBuilder('p')
            ->where('p.user = :user')
            ->andWhere('p.exercise = :exercise')
            ->andWhere('p.end IS NULL')
            ->orderBy('p.start', 'DESC')
            ->setParameters([
                'user' => $user,
                'exercise' => $exercise,
            ])
            ->getQuery()
            ->getResult();
    }