fluent nhibernate-映射列表会导致nullreferenceexception?

我有以下类和流畅的映射:

  public class A {
    public virtual int Id { get; private set; }
    public virtual string MyString { get; set; }
    public virtual IList<B> MyChildren { get; set; }
 }

  public class B {
    public virtual int Id { get; private set; }
    public virtual DateTime TheDate { get; set; }
  }

  public sealed class AMap : ClassMap<A> {
    public AMap() {
      Id(x => x.Id).GeneratedBy.Native().UnsavedValue(0);
      Map(x => x.MyString);
      HasMany(x => x.MyChildren).AsList(x => x.Column("Ordinal")).KeyColumn("AId").Not.KeyNullable();
    }
  }

  public sealed class BMap : ClassMap<B> {
    public BMap() {
      Id(x => x.Id).GeneratedBy.Native().UnsavedValue(0);
      Map(x => x.TheDate);
    }
  }

这将导致以下映射:

  <class xmlns="urn:nhibernate-mapping-2.2" mutable="true" name="A" table="`A`">
    <id name="Id" type="System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" unsaved-value="0">
      <column name="Id" />
      <generator class="native" />
    </id>
    <property name="MyString" type="AnsiString">
      <column name="MyString" length="150" not-null="true" />
    </property>
    <list name="MyChildren" mutable="true">
      <key not-null="true">
        <column name="AId" />
      </key>
      <index>
        <column name="Ordinal" />
      </index>
      <one-to-many class="B" />
    </list>
  </class>

但当我实际尝试保存的实例 NullReferenceException :

System.NullReferenceException : Object reference not set to an instance of an object.
at NHibernate.Collection.PersistentList.GetSnapshot(ICollectionPersister persister)
at NHibernate.Engine.CollectionEntry..ctor(ICollectionPersister persister, IPersistentCollection collection)
at NHibernate.Engine.StatefulPersistenceContext.AddNewCollection(ICollectionPersister persister, IPersistentCollection collection)
at NHibernate.Event.Default.WrapVisitor.ProcessArrayOrNewCollection(Object collection, CollectionType collectionType)
at NHibernate.Event.Default.WrapVisitor.ProcessCollection(Object collection, CollectionType collectionType)
at NHibernate.Event.Default.AbstractVisitor.ProcessValue(Object value, IType type)
at NHibernate.Event.Default.WrapVisitor.ProcessValue(Int32 i, Object[] values, IType[] types)
at NHibernate.Event.Default.AbstractVisitor.ProcessEntityPropertyValues(Object[] values, IType[] types)
at NHibernate.Event.Default.AbstractSaveEventListener.VisitCollectionsBeforeSave(Object entity, Object id, Object[] values, IType[] types, IEventSource source)
at NHibernate.Event.Default.AbstractSaveEventListener.PerformSaveOrReplicate(Object entity, EntityKey key, IEntityPersister persister, Boolean useIdentityColumn, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
at NHibernate.Event.Default.AbstractSaveEventListener.PerformSave(Object entity, Object id, IEntityPersister persister, Boolean useIdentityColumn, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
at NHibernate.Event.Default.AbstractSaveEventListener.SaveWithGeneratedId(Object entity, String entityName, Object anything, IEventSource source, Boolean requiresImmediateIdAccess)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveEventListener.SaveWithGeneratedOrRequestedId(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.EntityIsTransient(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveEventListener.PerformSaveOrUpdate(SaveOrUpdateEvent event)
at NHibernate.Event.Default.DefaultSaveOrUpdateEventListener.OnSaveOrUpdate(SaveOrUpdateEvent event)
at NHibernate.Impl.SessionImpl.FireSave(SaveOrUpdateEvent event)
at NHibernate.Impl.SessionImpl.Save(Object obj)

我在这里做错什么了?

澄清:这就是我如何使用这些课程:

var a = new A { MyChildren = new List<B> { new B { TheDate = DateTime.Now } } };
a.MyChildren[0].Parent = a;

session.Save(a);