nHibernate 从子子类中预先加载关系

Question

假设我有以下类定义：

public class Order
{
    public virtual int Id { get; protected set; }
    public virtual IList<OrderItemBase> OrderItems { get; set; }
}

public class OrderItemBase
{
    public virtual int Id { get; protected set; }
    public virtual string OrderRef { get; set; }
}

public class ProductOrder : OrderItemBase
{
    public virtual Product Product { get; set; }
}

public class OtherOrder : OrderItemBase
{
    public virtual string MiscData { get; set; }
}

public class Product
{
    public virtual int Id { get; protected set; }
    public virtual string Name { get; set; }
}

映射如下：

public class OrderMap : ClassMap<Order>
{
    this.Id(x => x.Id);
    this.HasMany(x => x.OrderItems).Inverse().KeyColumn("OrderId").Cascade.All();
}

public class OrderItemMap : ClassMap<OrderItembase>
{
    this.Id(x => x.Id);
    this.Map(x => x.OrderRef);
    this.DiscriminateSubClassesOnColumn("OrderTypeId");
}

public class ProductOrderMap : ClassMap<ProductOrder>
{
    this.DiscriminatorValue(1);
    this.References(x => x.Product).Column("ProductId");
}

public class OtherOrderMap : ClassMap<OtherOrder>
{
    this.DiscriminatorValue(2);
    this.Map(x => x.MiscData);
}

public class ProductMap : ClassMap<Product>
{
    // ... can't be arsed to type more code in here, you get the idea
}

现在提出问题：

当我为订单存储库编写查询时，我需要一些方法来预先加载所有OrderItems：

return this.All()
    .Where(o => o.Id == id)
    .FetchMany(o => o.OrderItems)

以及一些还加载相关 ProductOrder 产品项目的方法：

return this.All()
    .Where(o => o.Id == id)
    .FetchMany(o => o.OrderItems) // Ok so far...
    .ThenFetch(oi => oi.Product) // Uh-oh, the base order item doesn't have Product

如何让子类项目立即加载其关系，但仅在我需要时才加载？