How does Hibernate keep track of which objects are involved in a one to one association. I have a one to one association with an Instance object and a CategoryTree object.

The mapping is pretty straightforward:
<one-to-one name="categoryTreeRoot" />

Right now there are about 6 CategoryTree objects persisted. When I get the categoryTreeRoot from the Instance, how does Hibernate know which CategoryTree object to get? I know that it gets the correct one, but I'm not sure how. There don't seem to be any tables or columns that link the two things together. Can someone shed some light on this for me, or at least give me a RTFM and a section number?

The entrys in the two table have the same primary key on a standard one-to-one.

Ah! I see. So if I set the Instance's categoryTreeRoot to another CategoryTree, it won't work, because the primary key won't change. Would a foreign key restraint be more appropriate?

I meant constraint, not restraint. It's been a long day...

Of course, Hibernate will update the primary key if you change the association :) This is quite correct, as there may allways be just only one element can be part of the assoication on every side.

I'm a bit confused by the one-to-one part of the documentation. It says "if you want two objects to be related by a primary key association, you must make sure that they are assigned the same identifier value! "

Alright... so I don't want to make sure that they are assigned the same identifier value. The primary keys are just IDs here, and I don't want to change them if the association changes.

The next few lines down read:



Why did they use a many-to-one here? Do I need to use a many-to-one to describe my one-to-one?

This is just for the a many-to-one looks exactly the same in the java code as a one-to-one. Only a one-to-many will manifest itself as a java collection.

Maybe I need to explain my situation better. Looking back on my first post I realize that I didn't do such a good job of explaining.

The Instance object has a field of type CategoryTree called categoryTreeRoot. There is only ever 1 Instance object. There will be many CategoryTree objects, but the Instance is only ever associated with 1 of the CategoryTree objects. The rest of the CategoryTree objects are associated with eachother. The CategoryTree is more of a treenode type of object, so the name is a little misleading.

Is one-to-one the best way to go? Do I have to map from both ends of the association?

I just found it odd that they would throw something in about many-to-one in the documentation in the one-to-one section. As a beginner, it's a little confusing to me.

Okay, I got it figured out now. Thanks for your help, gloeglm.

This is the working mapping:

<many-to-one name="categoryTreeRoot" class="md.search.CategoryTree" column="categoryRoot"/>