如何在JPA中實現複雜的多對多關係？

Question

這裏的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; 

該機型

@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; 
} 

而對於一些原因，我不斷收到錯誤

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

任何人都可以指出我哪裏出了問題？謝謝

Answer 1

首先OrderDetailPK必須執行Serializable。

對於第二個請指定您將使用哪個ID，因爲您已指定product_id和order_id列爲insertable=false, updatable=false（只讀）。

所以你需要嘗試像下面這樣：

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

的更多信息，你可以在這裏找到：

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

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

Answer 2

從EmbeddedId的javadoc：

不支持在嵌入式id類中定義的關係映射。

所以你不能這樣做。我不認爲JPA 1指定了一個標準的方式來實現這個（在JPA 2中有@MapsId，但我從來沒有嘗試過），但這是我通常做的，大多數實現（我認爲至少Hibernate，EclipseLink和OpenJPA）都支持它：

使用原始類型聲明的主鍵類：

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

    public OrderDetailPK() {} 

    ... 
} 

標註您的實體@IdClass並聲明使用相同名稱的字段，但所需的類型：

@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; 

    ... 
} 

（我一直保持了@Id在實體的領域，但我沒有重新檢查，如果他們是強制性的）

Answer 3

當我這樣做之前（詳細this question and answer），我做了可嵌入的ID原語中的字段（對應的ID字段所涉及的實體），然後在實體中使用@MapsId。我相信這是滿足所有要求的最簡單的（並且我敢說是正確的）：實體中的字段是關係，ID類中的字段是原始字段，每個字段只准確映射一次（@MapsId字段不是真的是映射，但種類別）。

應用，爲您的情況下，ID類的樣子：

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

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

與實體類的樣子：

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() {} 
}