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merging a detached or new entity with an existing entity in hibernate/jpa best practice question
public User add(User user){

    User existingUser = getUserDao().findByBusinessKey(user.getBusinessKey(), false);
    user.setId(existingUser.getId());

    user = getUserDao().merge(user);

    return user;
}

if (user.getId() == null)
  em.persist(user);
else
  user = em.merge(user);

User user = userDao.findBySomething(Criteria c);

//stuff that modifies user

user = userDao.merge(user);