With NHibernate, how can I create an INHibernateProxy?

Question

After lots of reading about serialization, I've decided to try to create DTOs. After more reading, I decided to use AutoMapper.

What I would like to do is transform the parent (easy enough) and transform the entity properties if they've been initialized, which I've done with ValueResolvers like below (I may try to make it generic once I get it fully working). This part works.

public class OrderItemResolver : ValueResolver<Order, OrderItem>
{
    protected override OrderItem ResolveCore(Order source)
    {
        // could also use NHibernateUtil.IsInitialized(source.OrderItem)
        if (source.OrderItem is NHibernate.Proxy.INHibernateProxy)
            return null;
        else
            return source.OrderItem;
        }
    }
}

When I transform the DTO back to an entity, for the entities that weren't initialized, I want to create a proxy so that if the entity wants to access it, it can. However, I can't figure out how to create a proxy. I'm using Castle if that's relevant.

I've tried a bunch of things with no luck. The below code is a mess, mainly because I've been trying things at random without knowing what I should be doing. Anybody have any suggestions?

public class OrderItemDTOResolver : ValueResolver<OrderDTO, OrderItem>
{
    protected override OrderItem ResolveCore(OrderDTO source)
    {
        if (source.OrderItem == null)
        {
            //OrderItem OrderItem = new ProxyGenerator().CreateClassProxy<OrderItem>(); // Castle.Core.Interceptor.

            //OrderItem OrderItem = new ProxyGenerator().CreateClassProxy<OrderItem>();
            //OrderItem.Id = source.OrderItemId;

            //OrderItem OrderItem = new OrderItem();
            //var proxy = new OrderItem() as INHibernateProxy;
            //var proxy = OrderItem as INHibernateProxy;
            //return (OrderItem)proxy.HibernateLazyInitializer
            //ILazyInitializer proxy = new LazyInitializer("OrderItem", OrderItem, source.OrderItemId, null, null, null, null);
            //return (OrderItem)proxy;
            //return (OrderItem)proxy.HibernateLazyInitializer.GetImplementation();

            //return OrderItem;

            IProxyTargetAccessor proxy = new Castle.Core.Interceptor.

            var initializer = new LazyInitializer("OrderItem", typeof(OrderItem), source.OrderItemId, null, null, null, null);
            //var proxyFactory = new SerializableProxyFactory{Interfaces = Interfaces, TargetSource = initializer, ProxyTargetType = IsClassProxy};

            //proxyFactory.AddAdvice(initializer);
            //object proxyInstance = proxyFactory.GetProxy();
            //return (INHibernateProxy) proxyInstance;
            return null;


            //OrderItem.Id = source.OrderItemId;
            //return OrderItem;
        }

        else
            return OrderItemDTO.Unmap(source.OrderItem);
    }
}

Thanks, Eric

Maybe I over complicated it. This seems to work. Anybody see any issues with it?

public class OrderItemDTOResolver : ValueResolver<OrderDTO, OrderItem>
{
    protected override OrderItem ResolveCore(OrderDTO source)
    {
        if (source.OrderItem == null)
            return NHibernateSessionManager.Instance.Session.GetISession().Load<OrderItem>(source.AgencyId);

        else
            return OrderItemDTO.Unmap(source.OrderItem);
    }
}

Answer 1

This may be one of those cases where the answer is "don't", or at least "you probably shouldn't". If you're mapping DTOs into NHibernate mapped objects directly you're not really using the mapped objects as domain objects, just as a fancy way to push data in and out of the database. This of course may be all you're after but having done this myself in the past I've found that it's problematic trying to use the same DTO data format in both directions. If you're going cross-process you've turned the service into a (difficult to maintain) CRUD layer. If you're in the same process you're doing unnecessary data shuffling with DTOs.

Sending DTOs out is fine, but consider projecting the data into a format more closely aligned with what the client actually needs. What you get back is better expressed in specific DTOs that express only the data needed to perform the actual action (Command objects, essentially). With a few automatic properties they're trivial to construct. You can then have a business method that performs the necessary action with only the necessary information, and that in a format suited to the action being performed. My primary use of AutoMapper (which does rock) these days is to translate incoming DTOs into types that domain methods can consume.

Also, public setters on mapped objects are undesirable because they allow the object to be manipulated by any code without any validation. This means any modification to them can leave them in an invalid state.

If you don't really care about the above (and it's not always applicable) the way you load individual instances does leave you open do doing many individual database loads which is a potential performance issue.