Hibernate : merge detatched instance data in to persistent instance

Question

Session session = HibernateUtil.getSessionFactory().openSession();
Transaction tx = session.beginTransaction();
Course c1 = (Course) session.get(Course.class, 1);
tx.commit();
session.close();
c1.setCategory("science");
c1.setFee("3000");

//C1 became detached instance here

session = HibernateUtil.getSessionFactory().openSession(); 
tx = session.beginTransaction();
Course c2 = (Course) session.get(Course.class, 1);
c2.setCategory("social");
c2.setRecommendedBook("Modern History");
session.merge(c1);
tx.commit();
session.close();

Lets say my initial db table data is
category = maths
fee=1000
recommendedBook= Maths magic

I thought the above code would copy uncommon fields from C1 to C2, and override common fields of C2 with C1. So expected result would be
category = science
fee=3000
recommendedBook= Modern History

But it simply copied entire C1 data into C2 and updated db with C1, and all my C2 data is lost.Actual Result is
category = science
fee=3000
recommendedBook= Maths magic
So its infact not merging, but fully overriding. How to get my expected result?

Answer 1

The behavior you see is expected. How could Hibernate know that some fields must be merged and some others must not? null is a valid values for all fields, and setting fields to null by merging them is a perfectly valid operation.

To achieve your result, merge the fields that you want to merge explicitely:

 Course c2 = (Course) session.get(Course.class, 1);
 c2.setCategory("social");
 c2.setRecommendedBook("Modern History");
 if (c1.getCategory != null) {
     c2.setCategory(c1.getCategory());
 }
 if (c1.getRecommendedBook() != null) {
     c2.setRecommendedBook(c1.getRecommendedBook());
 }
 ...