c# - 操作失败 : The relationship could not be changed because one or more of the foreign-key properties is non-nullable

Question

这些是我的实体:

public class Currency : Exchange
{
     public List<CurrencyPrice> CurrencyPrice { get; set; }
}

public class CurrencyPrice : Price
{
    public int CurrencyId { get; set; }
    [ForeignKey("CurrencyId")]
    public Currency Currency { get; set; }
}

我第一次将值插入数据库时​​一切正常，但如果我更改数据库中的任何值并尝试插入或更新记录，我会收到此错误:

The operation failed: The relationship could not be changed because one or more of the foreign-key properties is non-nullable. When a change is made to a relationship, the related foreign-key property is set to a null value. If the foreign-key does not support null values, a new relationship must be defined, the foreign-key property must be assigned another non-null value, or the unrelated object must be deleted.

这是我的插入代码:

var currency =
    UnitOfWork.Exchange.Get(x => x is Currency && x.ExchangeType.Id == exchangeTypeId) as Currency;
if (currency != null)
{
    CurrencyPrice lastPriceValue = UnitOfWork.CurrencyPrice.Last(x => x.CurrencyId == currency.Id);
    if (lastPriceValue == null || lastPriceValue.Value != price)
    {
        currency.CurrencyPrice = new List<CurrencyPrice>
            {
                new CurrencyPrice {Id = Guid.NewGuid().ToString(), EntryDate = DateTime.Now, Value = price,CurrencyId = currency.Id,Currency = currency}
            };
    }
    else
    {
        lastPriceValue.EntryDate = DateTime.Now;
    }

    UnitOfWork.Commit();
}

我搜索了 StackOverflow 并找到了 this .但我不明白我必须做什么。

Answer 1

我认为问题在于这个代码块:

currency.CurrencyPrice = new List<CurrencyPrice>
{
    new CurrencyPrice {Id = Guid.NewGuid().ToString(), EntryDate = DateTime.Now, Value = price,CurrencyId = currency.Id,Currency = currency}
};

在这里您创建了一个新的 CurrencyPrice 并将其分配给一个 Currency，从而孤立了分配给此 的 CurrencyPrice 对象之前的货币。该对象现在仍在数据库中，但没有父对象。因此，EntityFramework 想要将此 CurrencyPrice 上的父 ID 设置为 NULL，数据库会阻止这导致您引用的错误。我在 this answer 中发布了对此问题的更详细解释。 .

为防止这种情况，请在添加新的之前从数据库中删除旧的 CurrencyPrice。或者，由于 CurrencyPrice 对象似乎始终依赖于它们的 Currency 父对象，因此您可以考虑将其作为标识关系。我已经在 Implementing identifying relationships with EF4 解释了如何使用 EntityFramework 4 Model First 创建它们。 .我还没有使用过 Code First，但方法应该是相似的。