如何自己实现类似 HasManyThrough 的功能？

 public function scopeWatchersOf($query, Topic $topic)
    {
        return $query->join('watches', function ($join) use ($topic) {
            $join->on('watches.topic_id', '=', 'topics.id')
                ->where('watches.topic_id', '=', $topic->id);
        })
            ->join('users', 'users.id', '=', 'watches.user_id')
            ->select(['users.id', 'users.name', 'users.avatar']);
    }
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