// Illuminate\Database\Eloquent\Concerns\HasAttributes
protected function serializeDate(DateTimeInterface $date)
{
    return $date instanceof DateTimeImmutable ?
        CarbonImmutable::instance($date)->toJSON() :
        Carbon::instance($date)->toJSON();
}

// class Carbon\Traits\Converter
public function toJSON()
{
    return $this->toISOString();
}

// 新建一条数据 Post 的评论，save 将自动添加 post_id 字段
$comment = new Comment(['message' => 'A new comment.']);
$post = Post::find(1);
$post->comments()->save($comment);

// 保持多条记录
$post->comments()->saveMany([
    new Comment(['message' => 'A new comment.']),
    new Comment(['message' => 'Another new comment.']),
]);

// 更新后需要重新加载模型及其关联，才会加到 $post->comments 中
$post->refresh();

// 所有评论，包括新保存的评论...
$post->comments;

User::query()->whereIn(
    'id',
    UserRolePivot::query()
        ->select('user_id')
        ->where('role_id', $roleId)
)->get();
// SELECT * FROM user WHERE id IN( SELECT user_id FROM user_role_pivot WHERE role_id = $roleId)