public void manytoone() throws Exception {
Order order = new Order();
order.setNumber("order-1234");
em.persist(order);
Product product = new Product();
product.setName("monitor");
em.persist(product);
OrderDetail detail = new OrderDetail(order, product);
order.getOrderDetails().add(detail);
product.getOrderDetails().add(detail);
em.persist(detail);
em.flush();
em.clear();