java - 具有公共(public)外键的 Hibernate ManyToMany 关系

Question

我尝试解决几年前在这里提到的相同问题: Hibernate many to many with a composite key with one field shared on either side of the relationship

我有一个 Menu 和一个 Item 类，我想实现一个单向关系，菜单保存它包含的所有项目。

Menu and Item both have composite keys out of the merchant_id foreign key and an auto incremental itemId/menuId. (EER 图图像)

因为当我声明复合键时 Hibernate 无法检索自动生成的 Id，并且该 Id 在系统中是唯一的，所以我保存实体时没有额外的 embeddedId PKClass:

 @Entity
 @Table(name="ITEM")
 public class Item extends AbstractTimestampEntity{
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="itemId", nullable=false)
    private long itemId;
    @ManyToOne
    @JoinColumn(name="merchantId", nullable=false)
    private Merchant merchant;
    @Column(name="name", length=45)
    private String name;
    @Column(name="description" , length=200)
    private String description;
    @Column(name="price")
    private double price;

    public Item(){} // getters & setters

---

@Entity
@Table(name="MENU")
public class Menu {
    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    @Column(name="menuId", nullable=false)
    private long menuId;
    @ManyToOne
    @JoinColumn(name="merchantId", nullable=false)
    private Merchant merchant;
    @Column(name="name", length=45)
    private String name;
    @ManyToMany
    @JoinTable(name="MENU_ITEM", joinColumns = {
        @JoinColumn(name="menuId", nullable=false, updatable=false)},
        //@JoinColumn(name="merchant.merchantId", nullable=false, updatable=false)},
        inverseJoinColumns = { @JoinColumn(name="itemId", nullable=false, updatable=false)})
        //                     @JoinColumn(name="merchantId", nullable=false, updatable=false)})
    private List<Item> items = new ArrayList<Item>(); // constructor, getters & setters

正如您从注释代码中看到的那样，这就是我的问题所在。如何在不修改规范化数据库表的情况下最好地映射实体？ (他们需要有相同的商家才能在数据库中进行验证)

Answer 1

我想你可能想要这样的东西:

CREATE TABLE `MENU_ITEM` (
    `merchant_id` INT NOT NULL,
    `menu_id` INT NOT NULL,
    `item_id` INT NOT NULL,

    PRIMARY KEY (`merchant_id`, `menu_id`, `item_id`),

    INDEX `ix_menuitem_item` (`item_id`, `merchant_id`),
    INDEX `ix_menuitem_menu` (`menu_id`, `merchant_id`),
    INDEX `ix_menuitem_merchant` (`merchant_id`),

    CONSTRAINT `fk_menuitem_merchant`
        FOREIGN KEY (`merchant_id`)
        REFERENCES `merchant` (`id`),

    CONSTRAINT `fk_menuitem_menu`
        FOREIGN KEY (`menu_id`, `merchant_id`)
        REFERENCES `menu` (`id`, `merchant_id`),

    CONSTRAINT `fk_menuitem_item`
        FOREIGN KEY (`item_id`, `merchant_id`)
        REFERENCES `item` (`id`, `merchant_id`)
)

但是，不幸的是，这不可能。

一列可以在最多1外键中使用，在这种情况下MENU_ITEM.merchant_id被使用3次(最多2次, 删除 fk_menuitem_merchant).

因此，您可能需要等效的东西:

CREATE TABLE `MENU_ITEM` (
    `merchant_id` INT NOT NULL,
    `menu_id` INT NOT NULL,
    `menu_merchant_id` INT NOT NULL,
    `item_id` INT NOT NULL,
    `item_merchant_id` INT NOT NULL,

    PRIMARY KEY (`merchant_id`, `menu_id`, `item_id`),

    INDEX `ix_menuitem_item` (`item_id`, `merchant_id`),
    INDEX `ix_menuitem_menu` (`menu_id`, `merchant_id`),
    INDEX `ix_menuitem_merchant` (`merchant_id`),

    CONSTRAINT `fk_menuitem_merchant`
        FOREIGN KEY (`merchant_id`)
        REFERENCES `merchant` (`id`),

    CONSTRAINT `fk_menuitem_menu`
        FOREIGN KEY (`menu_id`, `menu_merchant_id`)
        REFERENCES `menu` (`id`, `merchant_id`),

    CONSTRAINT `fk_menuitem_item`
        FOREIGN KEY (`item_id`, `item_merchant_id`)
        REFERENCES `item` (`id`, `merchant_id`),

    CHECK (`merchant_id` = `menu_merchant_id`),
    CHECK (`merchant_id` = `item_merchant_id`)
)

但是，不幸的是，MySQL 不支持 CHECK。

如您所见，这不是 ORM 问题。

所以，你有两个选择:

1. 实现一些触发器来模拟CHECK ( see here )

2. 让应用程序进行检查:

   @Entity
   public class Menu
   {
       protected class ItemList extends AbstractList<Item>
       {
           protected ArrayList<Item> list;
   
           public ItemList()
           {
               super();
               list = new ArrayList<>();
           }
   
           public ItemList(Collection<? extends Item> c)
           {
               super();
               list = new ArrayList<>(c.size());
               addAll(c);
           }
   
           @Override
           public boolean add(Item item)
           {
               if(!Objects.equals(merchant, item.merchant))
               {
                   throw new IllegalArgumentException();
                   // or return false;
               }
   
               return list.add(item);
           }
   
           @Override
           public Item get(int index)
           {
               return list.get(index);
           }
   
           @Override
           public int size()
           {
               return list.size();
           }
       }
   
       @Id
       @GeneratedValue(strategy = GenerationType.IDENTITY)
       @Column(name = "id", nullable = false)
       protected long id;
   
       @ManyToOne
       @JoinColumn(name = "merchant_id", nullable = false)
       protected Merchant merchant;
   
       @Column(name = "name", length = 45)
       protected String name;
   
       @ManyToMany
       @JoinTable(name = "MENU_ITEM",
           joinColumns = @JoinColumn(name = "menu_id"),
           inverseJoinColumns = @JoinColumn(name = "item_id"))
       protected List<Item> items = new ItemList();
   
       public List<Item> getItems()
       {
           return items;
       }
   
       public void setItems(List<Item> items)
       {
           this.items = new ItemList(items);
       }
   }