NHibernate映射多态一对一关系执行多个sql语句

Question

在我的模型中，我有类似的东西：
Order有很多Payment Payment有零个或一个PaymentMethod PaymentMethod是抽象的，有一个Payment（循环引用），不能延迟加载。

当我进行如下查询时：

var orders = session.Query<Order>().FetchMany(x => x.Payments)
.ThenFetch(x => x.PaymentMethod).ToList();

NHibernate执行一个语句，就像预期连接所有三个表一样。然后它为每个Payment执行一个语句。

当我删除循环引用时，一切正常，所以我猜它与HasOne关系有关。

这会让表现非常糟糕。为什么NHibernate会执行这些额外的查询以及如何修复映射，因此只会执行一个sql语句？

以下是我的映射：

public class Order
{
    public virtual int Id { get; set; }
    public virtual string Text { get; set; }
    public virtual ICollection<Payment> Payments { get; set; }
}

public class Payment
{
    public virtual int Id { get; set; }
    public virtual Order Order { get; set; }
    public virtual PaymentMethod PaymentMethod { get; set; }
}

public class PaymentMethod
{
    public virtual int Id { get; set; }
    public virtual Payment Payment { get; set; }
}

public class CreditcardPaymentMethod : PaymentMethod
{
}

public class OrderMap : ClassMap<Order>
{
    public OrderMap()
    {
        Id(x => x.Id).GeneratedBy.Native();
        Map(x => x.Text);
        HasMany(x => x.Payments).KeyColumn("OrderId").Not.KeyNullable().Inverse().Cascade.All();
    }
}

public class PaymentMap : ClassMap<Payment>
{
    public PaymentMap()
    {
        Id(x => x.Id).GeneratedBy.Native();
        References(x => x.Order).Column("OrderId").Not.Nullable().Cascade.SaveUpdate();
        References(x => x.PaymentMethod).Column("PaymentMethodId").Not.LazyLoad().Not.Nullable().Cascade.All();
    }
}

public class PaymentMethodMap : ClassMap<PaymentMethod>
{
    public PaymentMethodMap()
    {
        Id(x => x.Id).GeneratedBy.Native();
        DiscriminateSubClassesOnColumn("Discriminator").Not.Nullable();
        HasOne(x => x.Payment).PropertyRef(x => x.PaymentMethod).ForeignKey("PaymentMethodId").Cascade.All();
    }
}

public class CreditcardPaymentMethodMap : SubclassMap<CreditcardPaymentMethod>
{
    public CreditcardPaymentMethodMap()
    {
        DiscriminatorValue("Creditcard");
    }
}