getBacklogKey example

        $em = $eventArgs->getEntityManager();
        $uow = $em->getUnitOfWork();

        $queue = [];
        // Entity deletions         foreach ($uow->getScheduledEntityDeletions() as $entity) {
            $backlog = $this->getBacklog($entity);
            if (!$backlog) {
                continue;
            }
            $queue[$this->getBacklogKey($backlog)] = $backlog;
        }

        // Entity Insertions         foreach ($uow->getScheduledEntityInsertions() as $entity) {
            $backlog = $this->getBacklog($entity);
            if (!$backlog || $backlog->entity_id === null) {
                continue;
            }
            $queue[$this->getBacklogKey($backlog)] = $backlog;
        }