Some background about this issue can be found in the Hibernate forum here:
The issue is that it's not possible with recent Hibernate 5.x versions to customize the unique key name on a many-to-one association with the unique-key attribute that has also a orphan delete cascading. (The upcoming attachment has a complete test case)
The following mapping ...
...causes a mapping Exception:
If the many-to-one mapping is changed to also include unique="true", the mapping exception goes away, but the specified unique key name "uk_nextid" is ignored and not used. Instead an auto-generated key name is used: