Question

这里是db模式

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;

模特

@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

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

有人能指出问题所在吗？感谢

Answer 1

当我之前这样做时（详见this question and answer），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@MapsId实体。我相信这是满足所有要求的最简单（并且我敢说是正确的）：实体中的字段是关系，ID类中的字段是原始的，每列只映射一次（{{1字段不是真正的映射，而是一些别名）。

将其应用于您的案例，ID类如下所示：

@MapsId

实体类看起来像：

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

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

Answer 2

首先，OrderDetailPK必须实施Serializable。

第二个请指定您要使用的ID，因为您已将列product_id和order_id指定为insertable=false, updatable=false（只读）。

所以你需要尝试以下内容：

@EmbeddedId
@AttributeOverrides({
        @AttributeOverride(name = "product_id",column = @Column(name = "product_id")),
        @AttributeOverride(name = "listingId",column= @Column(name = "order_id"))
})
private OrderDetailPK id;

您可以在此处找到更多信息：

http://docs.oracle.com/javaee/6/api/javax/persistence/EmbeddedId.html

http://docs.oracle.com/javaee/6/api/javax/persistence/AttributeOverride.html

Answer 3

来自EmbeddedId javadoc：

不支持嵌入式id类中定义的关系映射。

所以你不能这样做。我不认为JPA 1指定了实现它的标准方法（在JPA 2中有@MapsId但我从未尝试过），但这是我通常做的和大多数实现（我认为至少Hibernate，EclipseLink和OpenJPA）支持它：

使用基本类型声明主键类：

@Embeddable
public class OrderDetailPK implements Serializable
{
    private int product;
    private int order;

    public OrderDetailPK() {}

    ...
}

使用@IdClass为您的实体添加注释，并使用相同名称但所需类型声明字段：

@Entity
@IdClass(OrderDetailPK.class)
public class OrderDetail {
    @Id
    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="product_id", insertable=false, updatable=false)
    private Product product;

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

    ...
}

（我一直将@Id保留在实体的字段中，但我没有重新检查它们是否是强制性的）

如何在JPA中实现复杂的多对多关系？

3 个答案: