JPA: trouble removing a 1:M cascade ALL relationship from entity

Question

I have an entity A, with a OneToMany relationship to entity B, and cascade set to ALL.  Entity B has a ManyToOne relationship to A.  In my scenario I create a new instance of B to a detached instance of A, and I add it to the 1:M of A and set the M:1 on B.
Then I'd like to merge the changes on A using EntityManager.merge(A).  However, before I do that I need to remove a different (old) instance of B from A, and then merge my changes so that my new instance of B is persisted.
I've tried a couple different scenarios and get different results (none favorable):
1.  First lookup the instance of B I want to remove via EntityManager.find(id), and then call EM.remove(B).  And then I call EM.merge(A) on the instance of A with my new B.  I get the following IllegalArgumentException during the merge:
java.lang.IllegalArgumentException: Cannot fulfill merge operation: Relationship >>outputElementInstances<< of the target entity {com.sap.eventus.wire.entity.EvtWireInstance(id=85e83543-45d1-49cd-bb90-8688699ca0d4)} has been modified
The relationship it's talking about is actually a different (but related) one that was affected when the old instance of B was removed.  It seems reasonable that JPA has determined that my instance of A with the new B is no longer in sync with the database and is complaining that a relationship has been changed.
Any ways around that?
2. I also tried simply removing B from A's 1:M, and setting B's M:1 to null, and then merging in hopes that would trigger a delete on the old B.  However the old B is not removed.
I was expecting this to trigger a delete, however, I realize that there are other relationships from B to other objects (such as a 1:M cascade ALL) that I did not remove which I suspect might cause JPA to not delete the old B.  Is this true?
3. And finally I tried 1 & 2 together, first calling EM.remove(old B), and then removing the old B from A and then merging, which yields the following IllegalStateException:
java.lang.IllegalStateException: Relationship >>wireInstance<< of entity {com.sap.eventus.wire.entity.EvtElementInstance(id=08ca6253-1385-45dd-9173-978ee298b55f)} contains the new entity {com.sap.eventus.wire.entity.EvtWireInstance(id=c9fbd4e7-5575-4cc9-9b7d-2a8549c43574)}
Which is another different (but related) relationship off of object B.
Basically, after all this, my question is: what is the correct way to remove a 1:M relationship and add a new one?  Simple enough, however I do not want to remove the old 1:M, re-query to get the new updated parent object and then add the new 1:M, as it's a fairly involved in creating and adding the new 1:M and it must be done BEFORE removing the old 1:M.
Thanks!

Answer 1

Hi Derek,
I understand your task as follows:
You are having three entities A, B_old and B_new. Within one transaction you want to do the following:
- remove B_old from the database
- remove B_old from the 1:n relationship A.bs
- persist the B_new
- add B_new to the 1:n relationship A.bs
- set the n:1 relationship B_new.a to refer to A
well, I suggest that you do this without a merge:
- em.remove(B_old)  // you have to do this explicitly as JPA 1 does not have automatic orphan removal
- A.bs.remove(B_old)
- em.persist(B_new) // also, you have to make sure that the entities related to B_new are managed upon commit
- A.bs.add(B_new)
- B_new.a = A
I hope this works.
Regarding your observations:
1. The error message is not self-explaining. We have got to improve it ;-).
2. You have to remove the B_old explicitly as JPA 1 does not have automatic orphan removal. 
3. This message should be self-explaining. Maybe there is one of the relationships EvtOutputElementInstance.wireInstance or EvtInputElementInstance.wireInstance still referring B_new. You'll surely find out.
Regards,
Adrian