How to persist a nullable one-to-one child of an updated parent?

Question

We have the following case:
- Parent entity (Employee, table name=Employee) has a child entity (EmployeePosition, table name=EmployeePosition)
We strongly need the EmployeePosition to be saved in another table for other reasons
- Child Entity field in Employee:
@OneToOne(mappedby="employee")
private EmployeePosition employeePosition ;
Required Scenario:
1. Open an existing employee record has employeePosition=null --> We do that normally
2. Select a position for the employeePosition --> We do that normally
3. Merge Employee entity, and JPA persists the employeePosition AUTOMATICALLY --> Don't know what to do or which annotations to use
How to do step 3?
N.B.
We tried Cascade.ALL, but, it generates exception (like: primary key column is not found as a parameter for the SQL built by JPA; the SQL is SELECTing from EmployeePosition using Employee Primary Key as parameters and not EmployeePosition primary key), as the parent entity is being merged while the child entity is being persisted; apparently, not supported.
Thanks in advance

Answer 1

You should be able to just find() the Employee and set its position and commit.
If you are finding the Employee in a seperate transaction/EntityManager or serializing it, then you can use merge(), which should work.
Include your code, SQL log, and exception stack trace.