NHibernate：对象实例的标识符从x更改为y

Question

我遇到了nhibernate的情况。我有这段代码：

private void UnlinkProductsFromPriceAgreement(NameValueCollection collection, IUnitOfWork unitOfWork)
{
    // Get the price agreement id
    int priceAgreementId = collection.GetInt("Id");

    foreach (int productId in collection.GetIds("unlinkProductsList"))
    {
        // Get the product price
        IProductPrice productPrice =
            ProductPriceRepository.Find(p => p.PriceAgreement.Id == priceAgreementId && p.Product.ProductId == productId).FirstOrDefault();

        // Remove the product price
        ProductPriceRepository.Delete(productPrice);
    }
}

运行此代码时出现此异常：

Qe.Common.Core.Exceptions.PersistencyException ---> NHibernate.HibernateException: identifier of an instance of Qe.Vending.Core.DomainObjects.Product was altered from 210036 to 0
   at NHibernate.Event.Default.DefaultFlushEntityEventListener.CheckId(Object obj, IEntityPersister persister, Object id, EntityMode entityMode) in d:\CSharp\NH\NH\nhibernate\src\NHibernate\Event\Default\DefaultFlushEntityEventListener.cs:line 111
   at NHibernate.Event.Default.DefaultFlushEntityEventListener.GetValues(Object entity, EntityEntry entry, EntityMode entityMode, Boolean mightBeDirty, ISessionImplementor session) in d:\CSharp\NH\NH\nhibernate\src\NHibernate\Event\Default\DefaultFlushEntityEventListener.cs:line 84

这怎么可能？