两种方式都以多对多关系向对象添加数据，

Question

Consider this class:

@Entity(name = "ORDERS") 
public class Order {
   @Id 
   @Column(name = "ORDER_ID", nullable = false)
   @GeneratedValue(strategy = GenerationType.AUTO)
   private long orderId;

   @Column(name = "CUST_ID")
   private long custId;

   @Column(name = "TOTAL_PRICE", precision = 2)
   private double totPrice;

   @OneToOne(optional=false,cascade=CascadeType.ALL, mappedBy="order",
   targetEntity=Invoice.class)
   private Invoice invoice;

   @ManyToOne(optional=false)
   @JoinColumn(name="CUST_ID",referencedColumnName="CUST_ID")
   private Customer customer;

   @ManyToMany(fetch=FetchType.EAGER)
   @JoinTable(name="ORDER_DETAIL",
           joinColumns=
           @JoinColumn(name="ORDER_ID", referencedColumnName="ORDER_ID"),
     inverseJoinColumns=
           @JoinColumn(name="PROD_ID", referencedColumnName="PROD_ID")
   )
   private List<Product> productList;       
   ...............
   The other attributes and getters and setters goes here

And this one:

@Entity(name = "PRODUCT") 
public class Product {
   @Id
   @Column(name = "PROD_ID", nullable = false)
   @GeneratedValue(strategy = GenerationType.AUTO)
   private long prodId;

   @Column(name = "PROD_NAME", nullable = false,length = 50)
   private String prodName;

   @Column(name = "PROD_DESC", length = 200)
   private String prodDescription;

   @Column(name = "REGULAR_PRICE", precision = 2)
   private String price;

   @Column(name = "LAST_UPDATED_TIME")
   private Date updatedTime;
   @ManyToMany(mappedBy="productList",fetch=FetchType.EAGER)
   private List<Order> orderList;       
   ...............
   The other attributes and getters and setters goes here

}

Now, if I want to add a product to an order I just do this:

    EntityManagerFactory emFactory = Persistence.createEntityManagerFactory("something");
    EntityManager em = emFactory.createEntityManager();
    EntityTransaction trans = em.getTransaction();

    trans.begin();

    Order order = em.find(Order.class, 1);
    Product product = em.find(Product.class, 51);

    order.getProductList().add(product);

    trans.commit
    em.close();
    emFactory.close();

The above example works - I can see it in the database. If I instead do this:

 EntityManagerFactory emFactory = Persistence.createEntityManagerFactory("something");
    EntityManager em = emFactory.createEntityManager();
    EntityTransaction trans = em.getTransaction();

    trans.begin();

    Order order = em.find(Order.class, 1);
    Product product = em.find(Product.class, 51);

    product.getOrderList().add(order);

    trans.commit
    em.close();
    emFactory.close();

The database isn't updating in the above example. Shouldn't it work both ways? Is there something I should do?

Hank

Answer 1

First off, you should always set both sides of the relationship.

The reason it doesn't work when you set from only the Product side is that the relationship is owned by the Order side.

Answer 2

Try add cascade on your @ManyToMany option like this :

@ManyToMany(
        mappedBy="productList",
        fetch=FetchType.EAGER,
        cascade = {CascadeType.ALL}
)

See the Hibernate Documentation which is very clear on this issue