Hi guys! I have this class Tag that has a set of children Tag objects. A tag can be child of more than one tag. So the mapping is:

1. <class name="Tag" table="tag">
2. <id name="id">
3. <generator class="assigned" />
4. </id>
5. <version name="version" unsaved-value="null" />
6. <property name="name" />
7.
8. <set name="children" table="tag_relation" lazy="true" fetch="select" cascade="all-delete-orphan">
9. <key column="parentId" />
10. <many-to-many column="childId" class="Tag" />
11. </set>
12.
13. <set name="parents" table="tag_relation" inverse="true" lazy="true" fetch="select">
14. <key column="childId" />
15. <many-to-many column="parentId" class="Tag" />
16. </set>
17. </class>

However there is a problem when deleting a Tag. All children of the deleted Tag are also deleted because of the cascade option (normal behaviour), however the Tag won't be delete from it's parents sets (the tag_relation join table), so an association will remain with an inexisting object. If I also add cascade delete or all on the parents set then the parents will also be deleted when deleting the tag which is not right. The only solution now is: at delete fetch the parents and delete the object from each association before the actual delete. This seems like a stretch. Isn't there any cascading option to automatically do this?

It is not recommended to add a cascade to a many-to-many association. Why should the childs be deleted if they can be in another tag?

I see your point mmerder. What you say would be normally true. The cascade delete is a particularity of my situation.
As for the parents association, after some thoughts I think I can answer myself: It's not possible. If you look at the bigger picture rather than my particular situation, it kinda makes seance.
Hibernate can't tell (or it would be very expensive to find out) what objects have association with what are you deleting, and afterwards it can't know how the association is called and what to place instead of the deleted object (supposing null is not allowed). I'll stick to my current solution: before deleting I go into parent tags and delete my object from their child list.