public function has($key)
    {
        try {
            $ttlColumn = $this->columns['ttl'];

            return (bool) $this->db->from($this->table)
                    ->where($this->columns['key'])->eq($this->prefix . $key)
                    ->andWhere(function ($group) use ($ttlColumn) {
                        $group->where($ttlColumn)->eq(0)
                        ->orWhere($ttlColumn)->gt(time());
                    })
                    ->count();
        } catch (PDOException $e) {
            return false;
        }
    }