How to JOIN two Entities without Mapped Relationship


Often you don’t want to map all relationships in an entity model. Let’s have a look at this class diagram:

There is no mapping between PurchaseOrder and PurchaseOrderItem because there is no use case for this relationship. We want to read all orders or display all items of an order, but we never want to have all orders with all items.

We have a foreign key on the database that stores the PurchaseOrder ID on the PurchaseOrderItem table. So we map the purchaseOrderId in the PurchaseOrderItem entity:

public class PurchaseOrderItem {

    private Integer id;
    private Integer purchaseOrderId;
    private Product product;

// Getters/Setters and more ...

Now if we need to join the two tables anyway, for example, in an aggregation to get the sum of the item price, we can do that because of the JPA 2.1 extension JOIN ON:

SELECT NEW entity.PurchaseOrderInfo(, sum(i.product.price))
FROM PurchaseOrder p
JOIN PurchaseOrderItem i ON = i.purchaseOrderId

This is supported in EclipseLink and Hibernate >= 5.1.