JPA Hibernate merge performs insert instead of update

I have a simple test, where I am trying to update the object, but merge seems to be performing an insert instead of update.

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations = {"classpath:spring/app-context.xml","classpath:spring/testdb-context.xml"})
public class UserJPATest {
@Test
public void testUpdate() throws Exception {
    System.out.println("update");

    User entity = ObjectManager.USER_DAO.findById(3L);
    entity.setUsername("harryUpdate");

    ObjectManager.USER_DAO.update(entity);

    User selEntity = ObjectManager.USER_DAO.findById(3L);
    Assert.assertEquals(entity.getUsername(),selEntity.getUsername());
}

}

This is my update method

@Override
@Transactional(propagation= Propagation.REQUIRES_NEW)
public T update(T entity) throws Exception {
    try {
        T merged = entityManage开发者_JAVA百科r.merge(entity);
        return merged;
    } catch (Exception e) {
        e.printStackTrace();
        throw new Exception(e);
    }
}

Update to code

@Override
@Transactional(propagation= Propagation.REQUIRES_NEW)
public T update(T entity) throws Exception {
    try {
        T merged = null;
        BaseEntity baseEntity = null;
        if(entity instanceof BaseEntity){
            baseEntity = (BaseEntity)entity;
            merged = entityManager.find(entityClass, baseEntity.getId());
        }
        merged = entityManager.merge(entity);
        entityManager.flush();
        return merged;
    } catch (Exception e) {
        e.printStackTrace();
        throw new Exception(e);
    }
}

Now I get the following error Could not commit JPA transaction; nested exception is javax.persistence.RollbackException: Transaction marked as rollbackOnly

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I had a version column which was not set when seed data was inserted into database. Hence all the problems with update and delete

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Merge will update the row ONLY IF it founds the id in the database, so if you create a new object and forgot to set its ID to the same ID of the desired row, it will try to insert a new one. In case your database table doesn't have any unique field, probably this insert will be successful as there will be no sql error.

I was able to update the desired row once I set the new object with the ID of that row.