Many To Many Relationship JPA with Entity - jpa

Question

I have an issue trying to generate multiple relationship in JPA with three Entities.
Order
Product
Modifier
I have an Entity to handle the relationship many to many.
OrderProducts (order_id and product_id)
Contains the relationship of one order can have multiple products
OrderDetails (order_products_id and modifier_id)
Contains the id of the previous relationship Order-Products and the Id of the modifier which is a set of multiple values that can affect the price of the product.
Not quite sure how to handle this kind of relationship in JPA as I'm new to it.

Answer 1

You need a join entity with a composite key. You will need to research it further.
Your entities:
#Entity
#Table(name = "ordertable")
#Data
public class Order {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#OneToMany(mappedBy = "order")
#EqualsAndHashCode.Exclude
private Set<OrderProductModifier> products;
}
#Entity
#Table(name = "product")
#Data
public class Product {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#EqualsAndHashCode.Exclude
private BigDecimal unitPrice;
}
#Entity
#Table(name = "modifier")
#Data
public class Modifier {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
#EqualsAndHashCode.Exclude
private BigDecimal modifier;
}
And the entity that ties it all together will need to have the foreign keys for each of the above entities, as you have noted.
#Entity
#Table(name = "orderproductmodifier")
#Data
public class OrderProductModifier {
#EmbeddedId
private OrderProductModifierId id;
#MapsId("orderId")
#ManyToOne
#EqualsAndHashCode.Exclude
#ToString.Exclude
private Order order;
#MapsId("productId")
#ManyToOne
#EqualsAndHashCode.Exclude
private Product product;
#MapsId("modifierId")
#ManyToOne
#EqualsAndHashCode.Exclude
private Modifier modifier;
}
#SuppressWarnings("serial")
#Embeddable
#Data
public class OrderProductModifierId implements Serializable {
private Long orderId;
private Long productId;
private Long modifierId;
}
This is pretty simple to use:
private void run() {
EntityManagerFactory factory = Persistence.createEntityManagerFactory("UsersDB");
EntityManager em = factory.createEntityManager();
em.getTransaction().begin();
Product product = new Product();
product.setUnitPrice(BigDecimal.TEN);
em.persist(product);
Modifier modifier = new Modifier();
modifier.setModifier(new BigDecimal(".90"));
em.persist(modifier);
Order order = new Order();
em.persist(order);
OrderProductModifier opm = new OrderProductModifier();
opm.setId(new OrderProductModifierId());
opm.setOrder(order);
opm.setProduct(product);
opm.setModifier(modifier);
em.persist(opm);
em.getTransaction().commit();
em.clear();
Order o = em.createQuery("select o from Order o join fetch o.products where o.id = 1", Order.class).getSingleResult();
System.out.println("Order for " + o.getProducts());
System.out.println("Order cost " + o.getProducts().stream().map(p->p.getProduct().getUnitPrice().multiply(p.getModifier().getModifier()).doubleValue()).collect(Collectors.summingDouble(Double::doubleValue)));
}
The above query could be better, but that will give you something to work on.