em.merge(course); // course is a detached entity

I got the following exception:

org.hibernate.exception.ConstraintViolationException: could not insert: package.Course.

The exception means it tried to insert a new row for merge.

The detached entity was initially retrieved from database using

course = em.find(Course.class, 876L); // the course id is 876.

It has a valid id. (using debugger).

If I load the entity from database first, then merge it, it works.

em.find(Course.class, 876L);
em.merge(course);


I have been working on solving this for days. Any helps are appreciated.
Thanks
Dave