NHibernate, save null reference value in DB as 0 (zero)

Question

I found this example of a tuplizer to do save 0 when saving null relationships. This is needed since I'm working on a app on a legacy database schema.

I tried the tuplizer here: http://nhforge.org/blogs/nhibernate/archive/2011/01/28/how-to-use-0-instead-of-null-for-foreign-keys.aspx

In that example, I got a nullreferenceexception for the ProxyFactory. Then I found an update to the code here: https://bitbucket.org/jfromaniello/hotgazpachoeg/changeset/87ac41c473ae

However, that doesn't work for me either. In the last method, SetPropertyValues (described as dirty hack 3, used when reading an object from DB), I get a nullref exception on this part, if(typeof(IEntity), when reading a not related object (not a Sample)

My mapping is as follows (simplified):

   Table("ej_sample");
        Not.LazyLoad();
        Id(s => s.Id, "sampleID").GeneratedBy.Native();
        References<Sample>(s => s.ParentSample, "parentSampleID").NotFound.Ignore();

The parentSampleID column must be 0 when no such object exists.

I figured, I only have to do the dirty hacks on insert and update (possibly in my case only insert).

On insert, I want to create a fake proxy, but the code in [2] loads the entity from the db (possibly to use a Null object?!).

Insert dirty hack:

        public override object[] GetPropertyValuesToInsert(object entity, IDictionary mergeMap, ISessionImplementor session) {
        var values = base.GetPropertyValuesToInsert(entity, mergeMap, session);

        //dirty hack 1
        for(int i = 0; i < values.Length; i++) {
            if(values[i] == null && typeof(IEntity).IsAssignableFrom(getters[i].ReturnType)) {
                values[i] = ((ISession)session).Load(getters[i].ReturnType, 0);
            }
        }
        return values;
    }

I tried creating a fake proxy instead of doing the above:

        public override object[] GetPropertyValuesToInsert(object entity, IDictionary mergeMap, ISessionImplementor session) {
        var values = base.GetPropertyValuesToInsert(entity, mergeMap, session);

        //dirty hack 1
        for(int i = 0; i < values.Length; i++) {
            if(values[i] == null && typeof(IEntity).IsAssignableFrom(getters[i].ReturnType)) {
                //values[i] = ((ISession)session).Load(getters[i].ReturnType, 0);
                values[i] = CreateFakeProxy(i);
            }
        }
        return values;
    }

    private object CreateFakeProxy(int i) {
        object proxy;
        using(var sessionImplementor = _sessionFactory.OpenSession()) {
            proxy = _sessionFactory
                .GetEntityPersister(getters[i].ReturnType.FullName)
                .CreateProxy(0, (ISessionImplementor)sessionImplementor);
        }
        return proxy;
    }

Then I get a nullref exception on the _sessionfactory, which is set in the ctor:

        private readonly ISessionFactoryImplementor _sessionFactory;

    public NullableTuplizer(EntityMetamodel entityMetamodel, PersistentClass mappedEntity)
        : base(entityMetamodel, mappedEntity) {
            _sessionFactory = entityMetamodel.SessionFactory;

    }

Any ideas how to accomplish this?

Answer 1

The easy solution is to add the following preinsert and preupdate listener.

public class NullToZeroEventListener : AuditEventListener, IPreInsertEventListener, IPreUpdateEventListener
{
    public bool OnPreInsert(PreInsertEvent @event) {
        ZeroNullIds(@event.State, @event.Persister.PropertyNames);
        return false;
    }

    public bool OnPreUpdate(PreUpdateEvent @event) {
        ZeroNullIds(@event.State, @event.Persister.PropertyNames);
        return false;
    }

    protected internal void ZeroNullIds(Object[] state, string[] propertyNames) {
        for(int i = 0; i < propertyNames.Length; i++) {
            if(state[i] != null || propertyNames[i].EndsWith("ID")) continue;
            state[i] = 0;
        }
    }
}

In the mapping be sure to ignore 0 ids, for example: References<User>(s => s.User, "userID").NotFound.Ignore().LazyLoad();

In your sessionfactory, add a listener for both preinsert and preupdate events (first shown here):

           .ExposeConfiguration(c =>
            {
                if(!c.EventListeners.PreInsertEventListeners.Any()) {
                    c.AppendListeners(ListenerType.PreInsert, new IPreInsertEventListener[] { new NullToZeroEventListener() });
                }
            });