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The following method is supposed to update a row in a table by updating an object.

public void saveOrUpdate(final T data) throws CPDPersistenceException 
 final EntityManager em = getEntityManager();
 try 
 final EntityTransaction transaction = em.getTransaction();
 transaction.begin();
 em.merge(data);
 transaction.commit();
 
 catch (final PersistenceException e) 
 throw new CPDPersistenceException(e);

The updated object "data" passed to saveorUpdate() has the new data in it. After the transaction.commit(), though, data is updated in the table but there is one element of the Object - The ID - that is still OK in the table I am updating, but in another table it has been changed to null. Does anybody know how this might happen? Thanks for your time.

Add updatable=false in @joincolumn of your child.So it will not update your child when parent table updates.

If you do not want some other table to be updated by this merge you need to only use the "Cascade" that you need but not the Cascade.MERGE for that relation on your data class. Check this link for some more information about how cascading works with JPA and Hibernate. https://vladmihalcea.com/a-beginners-guide-to-jpa-and-hibernate-cascade-types/

If still you do not want to have that column updated to null (I guess that's the foreign key), then you need to make sure that the child data is also on the data element when you call this method.