@ManyToMany 休眠映射的问题

Question

我是网络开发的新手。我一直在尝试做一个电子商务平台，并使用product和wishlist之间的@ManyToMany映射，使用wishlist_product作为连接表。

添加后，我没有遇到错误，但数据未添加到连接表中。我已经尝试解决此问题 2 天了，但一直面临持续失败。我在下面附上了wishlist 和product 的代码。

@Entity
@Table(name = "wishlist")
public class Wishlist {

    @Id
    @Column(name = "username")
    /*
     * @GeneratedValue(generator = "gen")
     * 
     * @GenericGenerator(name = "gen", strategy = "foreign", parameters
     * = @Parameter(name = "property", value = "login"))
     */
    private String username;

    @ManyToMany
    @JoinTable(name = "wishlist_product", 
                joinColumns = @JoinColumn(name = "username"),
                inverseJoinColumns = @JoinColumn(name = "product_id"))
    private List<Electronics> product;

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@Table(name = "electronics")
@DiscriminatorColumn(name = "product_Type", discriminatorType = DiscriminatorType.STRING)
public class Electronics {

    @Id
    @GenericGenerator(name = "CamIdGenerator", strategy = "com.virtusa.neuralhack.vlx.IdGenerator.CameraIdGenerator")
    @GeneratedValue(generator = "CamIdGenerator")
    // @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "product_id")
    private String productId;

    private String askingPrice;

    @Column(name = "age")
    private String howOld;

    @Column(name = "model")
    private String model;

    @Column(name = "brand")
    private String brand;

    @Column(name = "description")
    private String description;

    transient private String email;

    @ManyToOne
    @JoinColumn(name = "username")
    private User_Login user;

    @ManyToMany
    @JoinTable(name = "wishlist_product", joinColumns = @JoinColumn(name = "product_id"), inverseJoinColumns = @JoinColumn(name = "username"))
    private List<Wishlist> wishlist;

public void addToWishlist(String email, String productId) {

    Session currentSession = manager.unwrap(Session.class);
    Wishlist wishlist = currentSession.get(Wishlist.class,email);
        
    //System.out.println(wishlist.getUser());
        
    Electronics product = currentSession.get(Electronics.class, productId);
    System.out.println(product);

    wishlist.addToWishlist(product);
        
    //product.addAWishlist(wishlist);
    //System.out.println(wishlist.getProduct().get(0));

    currentSession.saveOrUpdate(wishlist);
}

Answer 1

确保你在读写事务中但不是只读的。

还有一些建议：

• 不要将cascade = CascadeType.ALL 与多对多关联一起使用，因为CascadeType.ALL 包含CascadeType.REMOVE，在多对多的情况下会导致太多实体被删除
• 当您不需要特定顺序时，使用 Set 而不是 List 进行多对关联。

Answer 2

你应该这样做：

@Entity
@Table(name = "wishlist")
public class Wishlist {

    @Id
    @Column(name = "username")
    private String username;

    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "wishlist_product", 
                joinColumns = @JoinColumn(name = "username"),
                inverseJoinColumns = @JoinColumn(name = "product_id"))
    private List<Electronics> products;
    
    public void addProduct(Electronics product) {
       products.add( product );
       product.getWishlists().add( this );
    }

    public void removeProduct(Electronics product) {
      products.remove( product );
      product.getWishlists().remove( this );
    }
}


@Entity
@Table(name = "electronics")
public class Electronics {

   @Id
   @Column(name = "product_id")
   private String productId;
   
   @ManyToMany(mappedBy = "products")
   private List<Wishlist> wishlists;
}

1. 只有bidirectional @ManyToMany 的拥有方应由@JoinTable 注释映射。你应该在另一边使用mappedBy。

2. 如果您想传播持久状态和其他状态，您应该在拥有方设置cascade 属性。

3. 为了保持双方之间的同步性，最好提供辅助方法来添加或删除子实体。

您的addToWishlist 方法将如下所示：

public void addToWishlist(String email, String productId) {
   Session currentSession = manager.unwrap(Session.class);
   Wishlist wishlist = currentSession.get(Wishlist.class,email);
   Electronics product = currentSession.get(Electronics.class, productId);
   wishlist.addProduct(product);
   currentSession.saveOrUpdate(wishlist);
}