Mapping Extra Attribute in a Join Table JPA 2 - jpa-2.0

Question

I am trying to model this relationship following this link http://www.javaworld.com/javaworld/jw-01-2008/images/datamodel.gif
Its the usual Many to Many relationship between Order and Products but I dont know how to add the extra columns in the Join table.
#Entity
#Table(name = "Orders")
public class Order {
#ManyToMany(cascade = CascadeType.ALL)
#JoinTable(name = "ORDER_LINES", joinColumns = { #JoinColumn(name = "ORDER_ID") }, inverseJoinColumns = { #JoinColumn(name = "PROD_ID") })
private Set<Product> products;
}
#Entity
#Table(name="PRODUCTS")
public class Product {
#ManyToMany(mappedBy="products")
private Set<Order> orders;
}
How to add Join Table extra attribute in JPA 2.0?
Thanks

Answer 1

There is no concept of having additional persistent attribute in relation in JPA (2.0). That's why relation with property is actually intermediate entity.
From both Order and Product entities, you need one-to-many relationship to new entity. Because of bidirectional relationship, new entity will have many-to-one relationships to Order and Product.
You need to go for something like this (shows only relationships, id's and other mappings stripped away):
#Entity
#Table(name="order_item")
public class OrderItem {
#ManyToOne
private Order order;
#ManyToOne
private Product product;
}
#Entity
public class Order {
#OneToMany (mappedBy = "order")
private Set<OrderItem> orderItems;
}
#Entity
public class Product {
#OneToMany(mappedBy = "product")
private Set<OrderItem> orderItems;
}