休眠不可变值对象

customer = // get customer from the current hibernate session
// customer has a discount with database id of 1

到这里一切都很好。但如果我打电话:

discount = SpecialDiscount.create("10%");
customer.setDiscountTo(discount);

session.save(customer);
// customer has a discount with database id of 2 now

Hibernate如何更新ID为2的同一折扣行,即使我已将其设置为另一个折扣值对象?另外,我想通过分离前一个对象来防止“具有相同标识的对象已经存在”错误。你有什么建议?

// An entity
class Customer {
    // one-to-one mapped immutable value object
    SpecialDiscount discount;

    SpecialDiscount discount() {
        return SpecialDiscount.create(this.discount);
    }

    void setDiscountTo(SpecialDiscount discount) {
        this.discount = SpecialDiscount.create(discount);
    }
}