Fluent Nhibernate - 映射列表会导致 NullReferenceException？

Question

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);
    }
  }

这会导致 A 的以下映射：

  <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>

但是当我实际尝试保存 A 的实例时，我得到一个 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)

我在这里做错了什么？

澄清：这就是我使用这些类的方式：

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

session.Save(a);

Answer 1

您应该将 MyChildren = New List(); 添加到 A 的构造函数中

Answer 2

子集合的关键列不应该是“Id”（而不是“AId”）吗？我认为这就是你问题的根源......

Answer 3

我想通了。上面的示例实际上确实有效，但我试图重现的问题仍在运行，导致了相同的错误。抱歉...

问题是我们创建了这个 ChildList 类，我们为 MyChildren 属性返回了该类。它只是包装了 List（或者 NHibernate 用于持久列表的任何具体列表），但它负责在集合中添加或删除的任何实例上设置 Parent 属性。

显然，这会导致 NHibernate 在保存新实例时出现问题。返回一个正常的具体列表是可行的。