Hibernate NonUniqueObjectException many-to-many

Question

I've created many-to-many relationships:

@Entity
@Table(name = "Orders")
public class Order {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@NotNull
@Column(name = "Id", unique = true)
private int id;

@ManyToOne
@JoinColumn(name = "UserId")
private User user;

@ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.ALL)
@JoinTable(name = "order_products", joinColumns = {@JoinColumn(name = "Order_id")},
        inverseJoinColumns = {@JoinColumn(name = "Product_id")})
private List<Product> products;

Products:

@Entity
@Table(name = "products")
public class Product {

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
@NotNull
@Column(name = "Id", unique = true)
private int id;


@NotNull
@Size(min = 5, max = 30)
@Column(name = "Name")
private String name;


@NotNull
@Column(name = "Price")
private double price;

@ManyToMany(fetch = FetchType.EAGER, mappedBy = "products")
private List<Order> orders;

and sql code;

CREATE TABLE IF NOT EXISTS products (
  Id    INT PRIMARY KEY AUTO_INCREMENT,
  Name  VARCHAR(30) UNIQUE NOT NULL,
  Price DOUBLE             NOT NULL
);
CREATE TABLE IF NOT EXISTS orders (
  Id     INT PRIMARY KEY AUTO_INCREMENT,
  UserId INT,
  FOREIGN KEY (UserId) REFERENCES users (Id)
);
CREATE TABLE IF NOT EXISTS order_products (
  Order_id   INT,
  Product_id INT,
  FOREIGN KEY (Order_id) REFERENCES orders (Id),
  FOREIGN KEY (Product_id) REFERENCES products (Id)
)

When I insert an order with all different products, all works, but when I want to add in one order two same products, I have an error when executing code:

Session session = sessionFactory.openSession();
try {
    session.beginTransaction();
    session.save(order);
    session.getTransaction().commit();
    session.close();
} catch (RuntimeException e) {
    session.getTransaction().rollback();
    session.close();
    e.printStackTrace();
}

Error:

org.hibernate.NonUniqueObjectException: A different object with the same identifier value was already associated with the session : [org.training.ytaranau.model.Product#1]

What can be the reason?

Answer 1

Please check the uniq id that you provide or insert to Product Entity. it was the same id of product.

Answer 2

As mentioned before are you braking the uniqeness of the relation by adding the same objcet twice.

So as a solution you can make an association object like ProductOrder and add a quantity field.

@Entity
public class ProductOrder {
    @Id
    private long orderId;

    @Id
    private long productId;

    private int quantity;

    @ManyToOne
    @PrimaryKeyJoinColumn(name="ORDERID", referencedColumnName="ID")
    private Order order;

    @ManyToOne
    @PrimaryKeyJoinColumn(name="PRODUCTID", referencedColumnName="ID")
    private Product product;
}

Order {
    @OneToMany(mappedBy="order")
    private List<ProductOrder> prodctOrder;
}

Product {
    @OneToMany(mappedBy="product")
    private List<ProductOrder> prodctOrder;
}