Could someone please explain what inverse='true' means in UML terms? I get really confused about it... but I know I need to know about to get bidirectionality to work for xxx-to-many relations.

Thanks for any hints.

Read the parent/child relationship chapter in the reference documentation.

i did - still confused. perhaps I'm stupid, or something.

Read it again :)

I guess I'm confused about the concept of a "inverse side" when there isn't a natural parent/child concept.

For instance, if I have a Product that can be contained in many ProductGroup's and that the ProductGroups can hold many products, who is the inverse side? In my head, none, yet Hibernate forces me to pick one.

It depends on the use case. Sometimes I want to get a list of products within a product group. Other times I need the product groups that a certain product belongs to.

Hmprf... my head is spinning.

inverse define who is responsible of the association.
Change made on the side of the inverse="true" are not persisted

A simple example :

Schema:

User * -------> * Group

table user (id integer)
table group (id integer)
table user_group(user_id integer, group_id integer)

The groups set in User is not inverse, the users set in Group is inverse.

When you delete an User the association in the table user_group is also deleted ... however if you delete a Group the user_group association is not deleted because the users set in Group is inverse = true.

Actaully it does not. Maybe you are confusing the issue due to the Parent/Child docs. That doc is just showing how to setup a true parent-child relation. If your "child" can be persisted and referenced outside its assoication with your "parent" than that is not a true parent-child relationship, just a simple one-to-many association.

It is totally valid to to have a collection in Hibernate mapped as inverse="false" (thats why its an option and actually the default). Even for a many-to-many, both sides can be mapped as inverse="false" meaning the association can be manipulated from either end.

So in UML terms, inverse=true simply mean that the given end of the association is not navigable?

NO!

inverse="true" deals with SQL insertion/update of the relationship. It has absolutly no link to Object manipulation and navigation.

A(inverse=true) <-many-to-many-> B

means a.getBs().add(b) will not do any SQL insert in the relation table
but means b.getAs().add(a) will do.

As you can see I'm able to navigate from a to b (a.getBs()) and b to a (b.getAs())

So what will a.getBs().add(b) do?

Only object level manipulation. No DB, that's the purpose of inverse="true"

Usually, you want to keep your object model consistent so you do the previous order.

But... the doc says it can't be so, because of constrains violations?? What does this mean?

After the huge amount of questions on inverse="true", I wrote a small tutorial on the wiki. I probably left some errors. Please read and improve it.
http://www.hibernate.org/155.html

I did not talk about uml., it's intentional ;-)