ASP.NET MVC：模型更新上的验证错误（外键字段）

Question

我的模型PaymentRequest如下：

 public class PaymentRequest
 {
        [Key]
        public int Id { get; set; }
        .....
        .....
        [DefaultValue(PaymentStatus.WaitForPayment)]
        public PaymentStatus Status { get; set; }


        [Required]
        public virtual ApplicationUser RequestedBy { get; set; }
        [ForeignKey("RequestedBy")]
        public string RequestedById { get; set; }


    }

这里RequestedById是外键。当我想更新模型的Status字段时，如下所示：

    PaymentRequest paymentrequest = db.PaymentRequests.Find(id);

    if (paymentrequest == null)
    {
        return HttpNotFound();
    }

    db.Entry(paymentrequest).State = System.Data.Entity.EntityState.Modified;
    paymentrequest.Status = PaymentRequest.PaymentStatus.WaitForPayment;
    db.SaveChanges();

当我检查调试模式时，我面临有关RequestedBy的必需属性的验证错误。但是，当我手动设置ID字段如下所示，问题解决：

PaymentRequest paymentrequest = db.PaymentRequests.Find(id);
if (paymentrequest == null || paymentrequest.PaymentType != PaymentRequest.PaymentRequestType.Tankhah)
{
    return HttpNotFound();
}

db.Entry(paymentrequest).State = System.Data.Entity.EntityState.Modified;
paymentrequest.Status = PaymentRequest.PaymentStatus.WaitForPayment;
string uid = paymentrequest.RequestedById;// <-- fix lines
paymentrequest.RequestedById = uid;// <-- fix lines
db.SaveChanges();

我想知道导致验证错误的错误是什么？是关于virtual属性及其延迟加载行为还是我错过了其他内容？