How to JOIN two Entities without Mapped Relationship

Often you don’t want to map all relationships in an entity model (a blog post about this topic in detail will follow).

Let’s have a look at this class diagram:

PurchaseOrder

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

On the database we have for sure the foreign key that stores the PurchaseOrder ID on the PurchaseOrderItem table. So we just map the purchaseOrderId in the PurchaseOrderItem entity:

@Entity
public class PurchaseOrderItem {

    @Id
    @GeneratedValue
    private Integer id;
    private Integer purchaseOrderId;

    @ManyToOne
    private Product product;

Now if we need to join the two tables anyway for example in a 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(p.id, sum(i.product.price)) 
FROM PurchaseOrder p 
JOIN PurchaseOrderItem i ON p.id = i.purchaseOrderId 
GROUP BY p.id

This is supported in EclipseLink and Hibernate >= 5.1.