I have a screen where I can manage (add / edit /delete) Orders. Each order is related to a Customer. On the screen I also display a list of Orders. In the list I also display fields from other entities, including the Customer. I am using a DTO (flattened, screen specific view) to populate the list.

When I add a new Order I need to create a relationship with the Customer. Rather than obtain the associated Customer from the database (which seems wasteful) I am simply creating a new() Customer and setting its ID to the appropriate value (I know this from a drop down). The Order and the Customer relationship are persisted correctly. My problem is that for the new() Customer the data in the DTO is null (apart from the ID). This makes sense. A Customer with the appropriate ID and all null fields is in the first level cache.

I can display the correct data by evicting the new() Customer from the cache. My question however is what is the recommended way of obtaining entities that are going to be used to create relationships?

Bad idea. NHibernate will think you are attempting to create a new, transient entity, and it will try to insert it (unless you evict it). This will cause a database constraint violation (duplicate primary key).

You will get into other trouble even if you try to prevent NHibernate from considering it transient by the "eviction trick". Just load the Customer by its ID and assign it to your Order. Don't worry about the "wasteful" select to create the entity Customer entity. If you are really that concerned about minimizing the number of selects, use the 2nd level cache.

Thanks for the reply Nels.

Actually you are not correct regarding NHibernate trying to INSERT a new entity, at least with the mappings set up the way that I have them (and I use this technique in a lot of places).

I can live with the SELECT but when need I make associations to lots of different entities it does seem rather wasteful.

One option would be to use ISession.Load, since it will return a proxy (if you have lazy loading enabled) that will not hit the database when your entity is not present in the session. If you access any properties of that object, it will then be loaded transparently.

Your process would then be as follows:

1. Fetch the primary key from your select list.
2. Call ISession.Load(typeof(Customer), primaryKey)
3. Now, when/if you access properties of that customer proxy, it will be loaded from the database without any interaction from you.

Note that your nhibernate session must be left open for this lazy loading proxy behavior to work properly.