public function deleteSubscriptionsFor($userId, $chanIdList = null)
    {
        $conditions = [
            Field::SUBER_NAME => $this
                ->notificationService
                ->getSubscriberName($userId),
        ];

        if ($chanIdList) {
            $conditions[Field::CHAN_ID] = $chanIdList;
        }

        $this
            ->notificationService
            ->getBackend()
            ->fetchSubscriptions($conditions)
            ->delete()
        ;
    }