hibernate insert to a collection causes a delete then all the items in the collection to be inserted again

Question

I have a many to may relationship CohortGroup and Employee. Any time I insert an Employee into the CohortGroup hibernate deletes the group from the resolution table and ins

Answer 1

Load the object in persistence context and call merge if exist and save if not exist.

Employee emp = new Employee();
//set emp attribute and members.
Session session = ...
Transaction transaction = ...
Employee db = (Employee) session.get(Employee .class,emp.getId());
if(db != null)  session.merge(emp);
else  session.save(emp);
/*session.saveOrUpdate(emp); This will delete the join table and reinsert entry  
again*/
transaction.commit();
session.close();