如何在JPA中实现复杂的多对多关系?

问题描述：

这是数据库架构

CREATE TABLE Products
(
    id          INT NOT NULL AUTO_INCREMENT,
    category_id  INT NOT NULL,
    description VARCHAR(100),
    price       DECIMAL(10, 2) NOT NULL,
    PRIMARY KEY (id),
    FOREIGN KEY (category_id) REFERENCES Categories(id)
) ENGINE = INNODB;

CREATE TABLE Orders
(
    id           INT NOT NULL AUTO_INCREMENT,
    customer_id  INT NOT NULL,
    status       VARCHAR(20) NOT NULL,
    date_created TIMESTAMP DEFAULT CURRENT_TIMESTAMP,
    PRIMARY KEY (id),
    FOREIGN KEY (customer_id) REFERENCES Customers(id)
) ENGINE = INNODB;

CREATE TABLE OrderDetails
(
    product_id INT NOT NULL,
    order_id   INT NOT NULL,
    quantity   INT NOT NULL,
    subtotal   DECIMAL(10, 2) NOT NULL,
    PRIMARY KEY (product_id, order_id),
    FOREIGN KEY (product_id) REFERENCES Products(id),
    FOREIGN KEY (order_id)   REFERENCES Orders(id)
) ENGINE = INNODB;

模型

@Embeddable
public class OrderDetailPK
{
    private Product product;
    private Order order;

    public OrderDetailPK() {}

    public OrderDetailPK(Product product, Order order)
    {
        this.product = product;
        this.order   = order;
    }
}

public class OrderDetail {
    @EmbeddedId
    private OrderDetailPK id;

    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="product_id", insertable=false, updatable=false)
    private Product product;

    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="order_id", insertable=false, updatable=false)
    private Order order;

    private int quantity;
    private double subtotal;

    public OrderDetail() {}

    public OrderDetail(OrderDetailPK id, int quantity, double subtotal)
    {
        this.product  = id.getProduct();
        this.order    = id.getOrder();
        this.quantity = quantity;
        this.subtotal = subtotal;
    }
    // getters, setters
}

public class Product {
    @Id
    private int id;

    private String description;
    private double price;

    @ManyToOne
    @JoinColumn(name="category_id")
    private Category category;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "Products")
    private List<OrderDetail> orderDetail;
}

public class Order {
    @Id
    private int id;

    @ManyToOne
    @JoinColumn(name="customer_id")
    private Customer customer;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "Orders")
    private List<OrderDetail> orderDetail;
}

由于某些原因，我不断收到错误消息

And for some reasons I keep getting the error

Concrete type "class models.OrderDetail" with application identity does not declare any primary key fields.

有人可以指出问题出在哪里吗?谢谢

Could anyone point me out where the problem is ? Thanks

答

当我之前这样做时(如此问题和解答中所述)，我在可嵌入的ID原语中创建了字段(对应于所引用实体的ID字段)，然后使用

When i did this before (as detailed in this question and answer), i made the fields in the embeddable ID primitives (corresponding to the ID fields of the entities referred to), and then used @MapsId in the entity. I believe this is the simplest (and dare i say correct) of meeting all the requirements: that the fields in the entity are relationships, that the fields in the ID class are primitive, that every column is mapped exactly once (the @MapsId fields not really being mappings, but sort of aliases).

将其应用于您的案例，ID类如下:

Applying that to your case, the ID class looks like:

@Embeddable
public class OrderDetailPK {
    private final int productId;
    private final int orderId;

    public OrderDetailPK(int productId, int orderId) {
        this.productId = productId;
        this.orderId = orderId;
    }
}

实体类如下:

public class OrderDetail {
    @EmbeddedId
    private OrderDetailPK id;

    @ManyToOne(cascade = CascadeType.ALL)
    @MapsId("productId")
    private Product product;

    @ManyToOne(cascade = CascadeType.ALL)
    @MapsId("orderId")
    private Order order;

    private int quantity;
    private double subtotal;

    public OrderDetail(Product product, Order order, int quantity, double subtotal) {
        this.id = new OrderDetailPK(product.getId(), order.getId());
        this.product = product;
        this.order = order;
        this.quantity = quantity;
        this.subtotal = subtotal;
    }

    protected OrderDetail() {}
}

如何在JPA中实现复杂的多对多关系?

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