jpa - 使用eclipselink的JPA简单多对多

我有这个ER图：



那被翻译成这些类：

User.java

@Entity
@Table(name = "user")
@NamedQueries({
    @NamedQuery(name = "User.findAll", query = "SELECT u FROM User u")})
public class User implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Column(name = "name")
    private String name;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "user")
    private Collection<UserHasNotify> userHasNotifyCollection;

UserHasNotify.java

@Entity
@Table(name = "user_has_notify")
@NamedQueries({
    @NamedQuery(name = "UserHasNotify.findAll", query = "SELECT u FROM UserHasNotify u")})
public class UserHasNotify implements Serializable {
    private static final long serialVersionUID = 1L;
    @EmbeddedId
    protected UserHasNotifyPK userHasNotifyPK;
    @Column(name = "has_read")
    private String hasRead;
    @JoinColumn(name = "notify_id", referencedColumnName = "id", insertable = false, updatable = false)
    @ManyToOne(optional = false)
    private Notify notify;
    @JoinColumn(name = "user_id", referencedColumnName = "id", insertable = false, updatable = false)
    @ManyToOne(optional = false)
    private User user;

UserHasNotifyPK.java

@Embeddable
public class UserHasNotifyPK implements Serializable {
    @Basic(optional = false)
    @Column(name = "user_id")
    private int userId;
    @Basic(optional = false)
    @Column(name = "notify_id")
    private int notifyId;

通知.java

@Entity
@Table(name = "notify")
@NamedQueries({
    @NamedQuery(name = "Notify.findAll", query = "SELECT n FROM Notify n")})
public class Notify implements Serializable {
    private static final long serialVersionUID = 1L;
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Basic(optional = false)
    @Column(name = "id")
    private Integer id;
    @Column(name = "message")
    private String message;
    @OneToMany(cascade = CascadeType.ALL, mappedBy = "notify")
    private Collection<UserHasNotify> userHasNotifyCollection;

现在，我将添加一个实体User和Notify并在它们之间创建一个关系。
因此，我编写了以下代码段：

        User user = new User();
        user.setName("John");

        Notify notify = new Notify();
        notify.setMessage("Hello World");

        userFacade.create(user);
        notifyFacade.create(notify);

        UserHasNotify uhn = new UserHasNotify();
        uhn.setNotify(notify);
        uhn.setUser(user);
        uhn.setHasRead("ok");
        uhnFacade.create(uhn);

但我收到此错误：

Internal Exception: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'user_id' cannot be null
Error Code: 1048
Call: INSERT INTO user_has_notify (has_read, user_id, notify_id) VALUES (?, ?, ?)
    bind => [3 parameters bound]
Query: InsertObjectQuery(com.test.entity.UserHasNotify[ userHasNotifyPK=null ])

为什么？？？？？？？？？？？

最佳答案

错误的原因可能在于远程通讯。您正在使用外观的事实意味着您正在与后端进行远程通信。
这意味着您在user实例上设置的notify和uhn实例将被发送以保留在远程系统上，而本地实例则永远不会收到生成的ID。
要验证并解决此问题，可以扩展示例：
保存user和notify后，从后端获取它们。这应该返回具有现有ID的持久化实例。然后，您可以使用它们来存储您的uhn关系。
编辑：我已经错过了UserHasNotify是一个具有嵌入式ID的有状态关系的事实。在代码中，您永远不会设置此ID，因此提供程序会丢失它。
对于这种情况，我建议使用IdClass而不是嵌入式ID-映射更易读，因此您可能不会两次映射User和Notify关系-一次在嵌入式PK中，一次再次在实体中；）
看起来像这样：

public class UserHasNotifyPK implements Serializable {
    private Notify notify;
    private User user;
    ...
    }

。

@Entity
@Table(name = "user_has_notify")
@IdClass(UserHasNotifyPK.class)
@NamedQueries({
    @NamedQuery(name = "UserHasNotify.findAll", query = "SELECT u FROM UserHasNotify u")})
public class UserHasNotify implements Serializable {

    private static final long serialVersionUID = 1L;

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

    @Id
    @JoinColumn(name = "notify_id", referencedColumnName = "id", insertable = false, updatable = false)
    @ManyToOne(optional = false)
    private Notify notify;

    @Id
    @JoinColumn(name = "user_id", referencedColumnName = "id", insertable = false, updatable = false)
    @ManyToOne(optional = false)
    private User user;

然后，您可以再次尝试相同的测试。