Specifically, I need to store a map of Strings to Sets.

I've tried to wrap the Set with a custom type, that stores as a property, but the ID field doesn't work. (it doesn't exist in the original set, no surprise there)

I also tried writing a custom type (MapSetType.java) and abusing some static methods to manipulate rows in a separate table. This doesn't work because there is no sane way of setting the ID for the rows.

Could someone please suggest a better way of doing this?
Lay out some pseudo-code, and the matching xml mapping?