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)