为什么NHibernate会在ISession.Refresh期间抛出"GenericADOException:无法初始化集合"异常？

Question

我一直在经历一种奇怪的行为(至少对我而言)ISession.Refresh().

我有一个带有延迟加载的子集合的实体,以及一个访问此集合的只读属性,它们都包含在二级缓存中.ISession.Refresh()在将事务提交到DB后,我在长会话中使用以获取最新数据,并获得以下错误:

NHibernate.Exceptions.GenericADOException : could not initialize a collection: [Test.NUnit.DBTest.TestModel.ParentTestEntity.Children#d4251363-cf88-4684-b65a-9f330107afcf][SQL: SELECT children0_.ParentTestEntity_id as ParentTe4_1_, children0_.Id as Id1_, children0_.Id as Id42_0_, children0_.RowVersion as RowVersion42_0_, children0_.Parent_id as Parent3_42_0_ FROM "ChildTestEntity" children0_ WHERE children0_.ParentTestEntity_id=?]
  ----> System.NullReferenceException : Object reference not set to an instance of an object.
    at NHibernate.Loader.Loader.LoadCollection(ISessionImplementor session, Object id, IType type)
    at NHibernate.Loader.Collection.CollectionLoader.Initialize(Object id, ISessionImplementor session)
    at NHibernate.Persister.Collection.AbstractCollectionPersister.Initialize(Object key, ISessionImplementor session)
    at NHibernate.Event.Default.DefaultInitializeCollectionEventListener.OnInitializeCollection(InitializeCollectionEvent event)
    at NHibernate.Impl.SessionImpl.InitializeCollection(IPersistentCollection collection, Boolean writing)
    at NHibernate.Collection.AbstractPersistentCollection.Initialize(Boolean writing)
    at NHibernate.Collection.AbstractPersistentCollection.ReadSize()
    at NHibernate.Collection.PersistentBag.get_Count()
    DBTest\TestModel\EntiteTestCacheCollectionsParent.cs(25,0): at Test.NUnit.DBTest.TestModel.ParentTestEntity.get_Count()
    at (Object , GetterCallback )
    at NHibernate.Bytecode.Lightweight.AccessOptimizer.GetPropertyValues(Object target)
    at NHibernate.Tuple.Entity.PocoEntityTuplizer.GetPropertyValuesWithOptimizer(Object entity)
    at NHibernate.Tuple.Entity.PocoEntityTuplizer.GetPropertyValues(Object entity)
    at NHibernate.Persister.Entity.AbstractEntityPersister.GetPropertyValues(Object obj, EntityMode entityMode)
    at NHibernate.Event.Default.AbstractVisitor.Process(Object obj, IEntityPersister persister)
    at NHibernate.Event.Default.DefaultRefreshEventListener.OnRefresh(RefreshEvent event, IDictionary refreshedAlready)
    at NHibernate.Event.Default.DefaultRefreshEventListener.OnRefresh(RefreshEvent event)
    at NHibernate.Impl.SessionImpl.FireRefresh(RefreshEvent refreshEvent)
    at NHibernate.Impl.SessionImpl.Refresh(Object obj)
    DBTest\NHibernateBehaviorTests.cs(610,0): at Test.NUnit.DBTest.NHibernateBehaviorTests.Test()
    --NullReferenceException
    at NHibernate.Engine.Loading.CollectionLoadContext.AddCollectionToCache(LoadingCollectionEntry lce, ICollectionPersister persister)
    at NHibernate.Engine.Loading.CollectionLoadContext.EndLoadingCollection(LoadingCollectionEntry lce, ICollectionPersister persister)
    at NHibernate.Engine.Loading.CollectionLoadContext.EndLoadingCollections(ICollectionPersister persister, IList`1 matchedCollectionEntries)
    at NHibernate.Engine.Loading.CollectionLoadContext.EndLoadingCollections(ICollectionPersister persister)
    at NHibernate.Loader.Loader.EndCollectionLoad(Object resultSetId, ISessionImplementor session, ICollectionPersister collectionPersister)
    at NHibernate.Loader.Loader.InitializeEntitiesAndCollections(IList hydratedObjects, Object resultSetId, ISessionImplementor session, Boolean readOnly)
    at NHibernate.Loader.Loader.DoQuery(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
    at NHibernate.Loader.Loader.DoQueryAndInitializeNonLazyCollections(ISessionImplementor session, QueryParameters queryParameters, Boolean returnProxies)
    at NHibernate.Loader.Loader.LoadCollection(ISessionImplementor session, Object id, IType type)

这是一个使用简化模型显示问题的单元测试:

    [Test]
    public void Test()
    {
        ISession session1 = NHibernateHelper.SessionFactory.OpenSession();
        ISession session2 = NHibernateHelper.SessionFactory.OpenSession();

        // Create a new entity tree and persist it
        ParentTestEntity parentSession1 = new ParentTestEntity();
        parentSession1.AddChild(new ChildTestEntity());
        session1.Save(parentSession1);
        session1.Flush();

        // Load the saved object into another session
        ParentTestEntity parentSession2 = session2.Get<ParentTestEntity>(parentSession1.Id);
        session2.Refresh(parentSession2); // Throws here
    }

以下是涉及的实体:

public class ParentTestEntity
{
    public virtual Guid Id { get; private set; }
    public virtual long RowVersion { get; private set; }

    public virtual IList<ChildTestEntity> Children { get; protected set; }

    public ParentTestEntity()
    {
        this.Children = new List<ChildTestEntity>();
    }

    public virtual int Count
    {
        get
        {
            return Children.Count;
        }

        set { }
    }

    public virtual void AddChild(ChildTestEntity child)
    {
        if (this.Children == null)
        {
            this.Children = new List<ChildTestEntity>();
        }

        this.Children.Add(child);
        child.Parent = this;
    }
}

public class ChildTestEntity
{
    public virtual Guid Id { get; private set; }
    public virtual long RowVersion { get; private set; }

    public virtual ParentTestEntity Parent { get; set; }
}

他们的映射:

public class ParentTestEntityMap : ClassMap<ParentTestEntity>
{
    public ParentTestEntityMap()
    {
        Cache.ReadWrite();

        Id(x => x.Id)
            .GeneratedBy.GuidComb();

        Version(x => x.RowVersion);

        HasMany(x => x.Children)
            .Inverse()
            .Cascade.All()
            .Cache.ReadWrite();

        Map(x => x.Count);
    }
}

public class ChildTestEntityMap : ClassMap<ChildTestEntity>
{
    public ChildTestEntityMap()
    {
        Cache.ReadWrite();

        Id(x => x.Id)
            .GeneratedBy.GuidComb(); 

        Version(x => x.RowVersion);

        References(x => x.Parent)
            .Not.Nullable();
    }
}

在我的测试中,我发现:

• 删除Count属性的映射,
• 删除Cache.ReadWrite()映射,
• 之前枚举集合Refresh,

足以Refresh让它正常工作.

任何人都知道我可以做什么让刷新工作？

笔记:

• 我可以在NHibernate 2.1.2和3.1.0中重现这种行为,
• 我知道空的setter Count是丑陋的,它只是在实际模型中镜像实体的映射.

Answer 1

使用Load()替换来Get()解决这个问题。

这不是通用的解决方案，因为Load()语义略有不同，如果相应的行不存在，则会抛出异常。

不过，这在我们的架构中是一个可接受的约束，因为我们知道加载的实体存在于数据库中。

不过，任何解决方案Get()仍然受到欢迎。