Hibernate version:
2.1.7

This is more of a question than a problem or issue. I've read Hibernate in Action and the manual, but did not see a clear answer to this.

I've got a somewhat deep class hierarchy, with five classes A, B, C, D, E, F and G, such as:


Mapping documents:


Background/Requirements:
- Class G has a many-to-one reference to any A or subclass of A. However in reality only instances of class B and E will be referenced.
- I need to access a collection of Gs from instances of subclass B and E.
- Instances of B and E are a very small minority of the total instances - way less than one percent.

Question:
How much overhead does it cost to handle the non-existent collections for all subclasses of A? It will be somewhat messy if I have to create two variations of G, one for B and one for E.

Thank you - Richard

I don't understand your relationships. A to G is a one-to-many, but there are also one-to-many relations from B and E to G? Why not a many-to-many between A and G? And what are the "non-existent" collections?

Let me try to explain better.

Any A or subclass of C can theoretically have a collection of G's. The G's have a many-to-one relationship to an A or a subclass of A.

The issue is that here are millions of instances of D and F, but only a few thousand instances of B and E. Because they all inherit from A, A is the first choice for placing the collection of G's. Then both the B's and E's inherit the behaviour I will program into this collection. Otherwise I have to define two variations of G, one that has a many-to-one pointing at B and another tha ha a many-to-one pointing at E.

The other approach I'm starting to consider is to define a separate entity that owns the collection of G's. Then just have an optional one-to-one between either B or E and this new entity.

Any thoughts? Thanks - Richard

the only thing that will happen is that you will get an empty set for those who don't have any elements.

if you mark the set as lazy then it won't be more than a construction of a set, which isn't a big deal (IMHO)

this lazy set won't cost anything if you dont access it (where it will have to go check if there are some objects associated with it...this check performance will depend on your indexes)

Max, thank you for the response. I was hoping that was the case. Now I just need to figure out if creating that empty set is really "significant" to my application. I tend to think not - but its going to happen millions of times a day. Its certainly not elegant, but I do not see a really easy way of refactoring to avaoid this.

Thanks again - Richard

a million times is many ;)

Maybe you should consider NOT having a set, but use a query to get that information instead ?

Duh! Of course.

I was too focused on using the very nice collection semantics that hibernate promotes. I will add a utility method to my DAO and all will be fine.