使用JPA和Criteria API将通用类型联接

Question

i'm trying to map the following entity:

I have an Order entity that contains different types of OrderData(can also be shared between multiple Order entities), depending on the actual order(different produckts, product variants etc):

//Simplified example
@Entity
@IdClass(OrderPK.class)
@Table(name = "tablename"
public class Order<T extends OrderData> {
    @Id
    @ManyToOne
    @JoinColumn(name = "whatever")
    private T orderData;

    // Might be complicating stuff
    @Id
    @ManyToOne 
    @JoinColumn(name = "somecolumn")
    private Account buyerAccount;
    // ...
}

// OrderData base class
@Entity
@Table(name = "thatothertable"
@Inheritance(strategy=InheritanceType.JOINED)
public class OrderData {
    @Id
    @Column(name = "id")
    private Long id;

    // extra stuff
}

Now the problem is: how can i get this to join the right subclass of OrderData? I want to be able to write something like this: List<Order<CustomOrderData>> ordersWithCustomOrderData = this.orderDAO.findAllOrders(); and get all Order entities with CustomOrderDatas. Is there a way to achieve this?

So far my DAO codes looks like this:

public List<Order> findAllOrders() {
    CriteriaBuilder cb = this.em.getCriteriaBuilder();
    CriteriaQuery<Order> cq = cb.createQuery(Order.class);
    Root<Order> root = cq.from(Order.class):
    cq.select(root):
    return this.em.createQuery(cq).getResultList();
}

How would i have to change the DAO code to support the generic type? Is it actually possible? If not, is there any alternative design to achieve a comparable structure and functionability (Orders with different OrderDatas, ability to search for Orders with specific subclass of OrderData)?

// Not working pseudo code of what i want
public <T extends OrderData> List<Order<T>> findAllOrders() {
    CriteriaBuilder cb = this.em.getCriteriaBuilder();
    // With createQuery(Order.class) i will get CrtieriaQuery<Order>
    CriteriaQuery<Order<T>> cq = cb.createQuery(Order.class);
    // as above
    Root<Order<T>> root = cq.from(Order.class):
    cq.select(root):
    return this.em.createQuery(cq).getResultList();
}

Answer 1

1. Idea

Why do you need a generic Attribute? Generic entities are not possible in JPA but you could just put the reference to a Orderdata as:

private OrderData orderData;

2. Idea

You can save this enum in your entity and then compare OrderDataType.getClazz() to your OrderData1.getClass() and you can query for this types. If you are using JPA 2.1 you can save the full classname in the database column with a custom Fieldconverter and then query for the class.

public enum OrderDataType {
    Order1(Order1.class);

    private Class<? extends OrderData> clazz;

    private OrderDataType(Clazz<? extends OrderData> clazz) {
        this.clazz = clazz;
    }

    public Class<? extends OrderData> getClazz() {
        return clazz;
    }
}