如何在nhibernate中映射抽象类集合

我有以下代码

[Class]
[Discriminator(Column="MachineType",TypeType=typeof(string))]
public abstract class Machine
{
  [Property]
  public string Name{get;set;}
}

[Subclass(DiscriminatorValue="Heavy",ExtendsType=typeof(Machine))]
public class HeavyMachine : Machine
{
  [Property]
  public int Weight { get; set; }
}

[Subclass(DiscriminatorValue="Fast",ExtendsType=typeof(Machine))]
public class FastMachine : Machine
{
  [Property]
  public float Speed { get; set; }
}

[Class]
public class Module
{
    List<Machine> machines = new List<Machine>();

    [Bag(Name = "Machines", Cascade = "all", Lazy = false, Inverse=true)]
    [Key(1, Column = "Machine")]
    [OneToMany(2, ClassType = typeof(Machine))]
    public IList<Machine> Machines
    {
      get
      {
        return machines.AsReadOnly();
      }
      private set
      {
        machines = value.ToList();
  }
    }
}

使用上面的代码,我没有得到任何错误,但是在从数据库检索对象之后,模块中的机器集合仍然是空的。机器(及其子类)的映射似乎没有问题,因为正确返回了Machine类型的属性。

映射抽象类集合需要哪些Nhibernate.Mapping.Attributes?

提前谢谢!