hibernate - SchemaGen 错误地生成了不必要的关联表

Question

我有一个非常简单的 DB 模式，经典的 订单---(一对多)---> OrderItem <---(多对一)--- 产品
但是，当我使用 SchemaGen 生成 DDL 时，它会创建一个额外的层 orders_orders_item 和 product_orders_item ，实际上添加了一个冗余的关联表层:

create table orders (order_id bigint not null auto_increment, order_amout varchar(255), primary key (order_id)) engine=InnoDB;
create table orders_item (orders_item_id bigint not null auto_increment, order_amount integer, product_id bigint not null, primary key (orders_item_id)) engine=InnoDB;
create table orders_orders_item (OrderEntity_order_id bigint not null, orderItems_orders_item_id bigint not null) engine=InnoDB;
create table product (id bigint not null auto_increment, name varchar(255), price decimal(19,2), primary key (id)) engine=InnoDB;
create table product_orders_item (ProductEntity_id bigint not null, orders_orders_item_id bigint not null) engine=InnoDB;

似乎认为 之间存在多对多关联。订单和 订单项 ，以及 之间产品 和 orders_item .
为什么会这样？
我的实体类如下所示:
订单 :

@Entity
@Table(name = "orders")
public class OrderEntity {

    @Id
    @Column(name = "order_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @OneToMany
    private List<OrderItemEntity> orderItems;

    protected OrderEntity() {
    }

    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public List<OrderItemEntity> getOrderItems() {
        return orderItems;
    }
    public void setOrderItems(List<OrderItemEntity> orderItems) {
        this.orderItems = orderItems;
    }
}

订单项目:

@Entity
@Table(name = "orders_item")
public class OrderItemEntity {

    @Id
    @Column(name = "orders_item_id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @ManyToOne(optional = false, fetch = FetchType.EAGER)
    @JoinColumn(name = "product_id", nullable = false, updatable = false)
    private ProductEntity product = new ProductEntity();

    @Column(name = "order_amount")
    private int amount;

    protected OrderItemEntity() {
    }

    public OrderItemEntity(ProductEntity product, int amount) {
        super();
        this.product = product;
        this.amount = amount;
    }
    public int getAmount() {
        return amount;
    }
    public void setAmount(int amount) {
        this.amount = amount;
    }
    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public ProductEntity getProduct() {
        return product;
    }
    public void setProduct(ProductEntity product) {
        this.product = product;
    }
}

产品 :

@Entity
@Table(name = "product")
public class ProductEntity {

    @Id
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private long id;

    @OneToMany
    private List<OrderItemEntity> orders = new ArrayList<>();

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

    @Column(name = "price")
    private BigDecimal price;

    protected ProductEntity() {
    }

    public ProductEntity(String name) {
        this.name = name;
    }

    public ProductEntity(String name, BigDecimal price) {
        super();
        this.name = name;
        this.price = price;
    }

    public long getId() {
        return id;
    }
    public void setId(long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public BigDecimal getPrice() {
        return price;
    }
    public void setPrice(BigDecimal price) {
        this.price = price;
    }
}

Answer 1

orders_orders_item表:

由于您尚未指定 @JoinColumn ，而不是只有一个外键，而是创建了一个带有两个外键的额外表。

orders_orders_item (OrderEntity_order_id, orderItems_orders_item_id) 

要解决这个问题，您应该添加 @JoinColumn注释到 OrderEntity .

@Entity
@Table(name = "orders")
public class OrderEntity {

    //...

    @OneToMany
    @JoinColumn(name = "order_id")
    private List<OrderItemEntity> orderItems;
}

有了这个映射，只有一个 order_id列将添加到 orders_item table 。 orders_orders_item不会不必要地创建表

product_orders_item表:

orders_item之间存在双向关系和 product .您指定了 @JoinColumn在 orders_item边。这导致了 product_id 的创建 orders_item上的专栏表哪个是对的。
但另一方面，由于您没有指定 mappedBy它是一个双向关系 DB 试图通过创建 product_orders_item 来建立链接。 table 。
您应该添加 mappedBy归因于 orders field 。

@Entity
@Table(name = "product")
public class ProductEntity {

    // ...

    @OneToMany (mappedBy="product")
    private List<OrderItemEntity> orders = new ArrayList<>();
}

这说明已经在 product 之间创建了双向关系。和 orders_item (无需映射 orders 字段)。不需要额外的表创建等。感谢 @JoinColumn , product_id列是在 orders_item 中创建的 table 。