I'm not sure how much this affects this particular problem, but I'm getting a NonUniqueObjectException "a different object with the same identifier value was already associated with the session" when it gets to the session.lock() call in MergePersistentMethod. merge() doesn't update the entity parameter, it returns the updated instance, so

session.merge(target);
session.lock(target, LockMode.NONE);
...
return target;

should be

Object merged = session.merge(target);
session.lock(merged, LockMode.NONE);
...
return merged;

but this wouldn't work for Grails since it's an instance method.

The bug can also be reproduced when working with entities mapped in GORM.

I have changed the JPA/Hibernate annotated java classes and moved them to grails-app/domain. I experience the same behavior.

Please see BugProof2.zip

The merge() method has been changed to return the new merged instance, so you have to write code like this:

def b = book.merge()

There is an argument that maybe this should be a static method, if there is a massive outcry we can make a static version of the method