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

From the EmbeddedId javadoc:

Relationship mappings defined within an embedded id class are not supported.

So you cannot do it this way. I don't think JPA 1 specifies a standard way to implement this (in JPA 2 there is @MapsId but I never tried), but this is what I usually do and most implementations (I think at least Hibernate, EclipseLink and OpenJPA) support it:

Declare your primary key class using primitive types:

@Embeddable
public class OrderDetailPK implements Serializable
{
    private int product;
    private int order;

    public OrderDetailPK() {}

    ...
}

Annotate your entity with @IdClass and declare the fields using the same name but the desired types:

@Entity
@IdClass(OrderDetailPK.class)
public class OrderDetail {
    @Id
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="product_id", insertable=false, updatable=false)
    private Product product;

    @Id
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="order_id", insertable=false, updatable=false)
    private Order order;

    ...
}

(I have always kept the @Id on the fields in the entity but I didn't recheck if they are mandatory)

Answer 2

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() {}
}

Answer 3

First of all OrderDetailPK has to implement Serializable.

For second please specify which ID's you are going to use, because you has specified columns product_id and order_id as insertable=false, updatable=false (read-only).

So you need to try something like the following:

@EmbeddedId
@AttributeOverrides({
        @AttributeOverride(name = "product_id",column = @Column(name = "product_id")),
        @AttributeOverride(name = "listingId",column= @Column(name = "order_id"))
})
private OrderDetailPK id;

More information you may find here:

http://docs.oracle.com/javaee/6/api/javax/persistence/EmbeddedId.html

http://docs.oracle.com/javaee/6/api/javax/persistence/AttributeOverride.html