减去话题回复数需要加个 reply_count 是否为 0 的判断？

public function deleted(Reply $reply)
{
        if ($reply->topic->reply_count > 0) {
            $reply->topic->decrement('reply_count', 1);
        }
}

否则容易报错：

Illuminate \ Database \ QueryException (22003)
SQLSTATE[22003]: Numeric value out of range: 1690 BIGINT UNSIGNED value is out of range in '(`larabbs`.`topics`.`reply_count` - 1)' (SQL: update `topics` set `reply_count` = `reply_count` - 1, `updated_at` = 2018-06-08 16:39:14 where `id` = 63)