现在有Topic, Post, Comment三个模型, Comment相对Topic和Post是一对多的关系, 也就是说每个Topic和Post都可以有多条Comment.
但现在尝试插入Comment时, 发现并没有插入对应的commentable_id和commentable_type???
报错如下:

SQLSTATE[HY000]: General error: 1364 Field 'commentable_id' doesn't have a default value (SQL: insert into `comments` (`content`, `updated_at`, `created_at`)

App\Models\Comment

public function commentable()
{
    return $this->morphTo();
}

App\Models\Topic

public function comments()
{
    return $this->morphMany('App\Models\Comment', 'commentable');
}

App\Models\Post

public function comments()
{
    return $this->morphMany('App\Models\Comment', 'commentable');
}

Comments数据表中有$table->morphs('commentable');.

请问问题到底出在哪里?`