現在有Topic, Post, Comment三個模型, Comment相對Topic和Post是一對多的關係, 也就是說每個Topic和Post都可以有多條Comment.
但現在嘗試插入Comment時, 發現並沒有插入對應的commentable_id和commentable_type#???
報錯如下:

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資料表中有$table->morphs('commentable');.

請問問題到底出在哪裡?`