为什么BelongsTo()不能在Laravel 6中工作？

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Post extends Model
{
    // 其他属性和方法
}

namespace App\Models;

use Illuminate\Database\Eloquent\Model;

class Comment extends Model
{
    public function post()
    {
        return $this->belongsTo(Post::class, 'post_id');
    }
}

use Illuminate\Database\Migrations\Migration;
use Illuminate\Database\Schema\Blueprint;
use Illuminate\Support\Facades\Schema;

class CreateCommentsTable extends Migration
{
    public function up()
    {
        Schema::create('comments', function (Blueprint $table) {
            $table->id();
            $table->unsignedBigInteger('post_id');
            $table->text('content');
            $table->timestamps();

            $table->foreign('post_id')->references('id')->on('posts');
        });
    }

    public function down()
    {
        Schema::dropIfExists('comments');
    }
}