How to implement a complex many to many relationship in JPA?

Question

Here the db schema

CREATE TABLE Products
(
    id          INT NOT NULL AUTO_INCREMENT,
    category_id  INT NOT NULL,
    description VARCHAR(100),
    pric         


        

Answer 1

When i did this before (as detailed in this question and answer), i made the fields in the embeddable ID primitives (corresponding to the ID fields of the entities referred to), and then used @MapsId in the entity. I believe this is the simplest (and dare i say correct) of meeting all the requirements: that the fields in the entity are relationships, that the fields in the ID class are primitive, that every column is mapped exactly once (the @MapsId fields not really being mappings, but sort of aliases).

Applying that to your case, the ID class looks like:

@Embeddable
public class OrderDetailPK {
    private final int productId;
    private final int orderId;

    public OrderDetailPK(int productId, int orderId) {
        this.productId = productId;
        this.orderId = orderId;
    }
}

And the entity class looks like:

public class OrderDetail {
    @EmbeddedId
    private OrderDetailPK id;

    @ManyToOne(cascade = CascadeType.ALL)
    @MapsId("productId")
    private Product product;

    @ManyToOne(cascade = CascadeType.ALL)
    @MapsId("orderId")
    private Order order;

    private int quantity;
    private double subtotal;

    public OrderDetail(Product product, Order order, int quantity, double subtotal) {
        this.id = new OrderDetailPK(product.getId(), order.getId());
        this.product = product;
        this.order = order;
        this.quantity = quantity;
        this.subtotal = subtotal;
    }

    protected OrderDetail() {}
}