Most of our collections contain entities with a boolean IsPrimary property. Very frequently we will have queries that want to select a parent entity and its primary child entity. We would like to redundantly expose the "Primary" child entity directly on the parent, to make both queries and code simpler.

However, I can't figure out how to map this situation. I tried setting this up as a one-to-one, but only one-to-many has a "where" attribute to filter down to the primary collection item.

Is this possible with NHibernate?

I can't think of how to achieve this with a mapping. However, there are two approaches I can think of to solve the problem:

1. Use a getter property to get the "primary" child entity by iterating through the child collection, looking for the item with the IsPrimary property = true. In this case, everything is handled by your domain logic.

2. Add a foreign key on your parent entity table to the child entity table, pointing to the primary child for any given parent entity record. This works quite nicely as you can just set your "PrimaryChild" property to the proper Child instance and NHibernate will persist to the database appropriately.

#1 won't be very useful because you can't use it in queries (since NHibernate doesn't know about the property). It also forces the (otherwise lazy-loaded) child collection to load.

I'd like to do #2, that would get rid of the problems of clearing the IsPrimary indicator on the old child when a new one becomes primary (and also preventing multiples from being marked primary). Unfortunately, we got into NHibernate after our base schema was virtually set in stone, and I doubt my bosses will go for changing the way we track primary children in the database :(

Did you ever resolve this problem?

We never resolved it, although if you have the same issue, I'd suggest solution #2. We just can't do it because it would require reworking the way we track primary children on dozens of tables in our application, and management will never go for it ...