java - 在 Hibernate 实体中使主键与外键相同

Question

我是 Hibernate 新手。我正在研究以下两个实体:

实体 1 如下:

@Entity
@Table(name = "vm_user")
public class VmUser implements Serializable {

    private static final long serialVersionUID = 1L;

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

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

    @Column(name = "created_date")
    private Instant createdDate;

    @Column(name = "last_modified_date")
    private Instant lastModifiedDate;

    @OneToOne
    @JoinColumn(unique = true)
    private User user;             <--- HOW WILL I DENOTE THIS PRIMARY KEY OF VMUSER ENTITY ?

在mysql的关联表中，即vm_user，user_id既是主键，也是外键，引用user表的id与 User 实体关联。

实体 2 如下:

@Entity
@Table(name = "my_entity")
public class MyEntity implements Serializable {

    private static final long serialVersionUID = 1L;

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

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

    @Column(name = "created_date")
    private Instant createdDate;

    @Column(name = "last_modified_date")
    private Instant lastModifiedDate;

    @ManyToOne
    private A a;

    @OneToOne
    @JoinColumn(unique = true)
    private B b;

在mysql的关联表中，即my_entity，主键是id of a和id of b的组合。我不知道如何在 Hibernate 实体 MyEntity 中表示这一点。

关于同样的事情，我浏览了几篇文章:Hibernate foreign key as part of primary key和 JPA & Hibernate - Composite primary key with foreign key ，但不知道如何做这两个？

Answer 1

解决方案是@MapsId

例如

@Entity
@Table(name = "vm_user")
public class VmUser implements Serializable {

    @Id
    @Column(name = "user_id")    
    private Integer id;

    @MapsId
    @OneToOne
    private User user;  

您还可以删除 @JoinColumn(unique = true)，因为 @Id 已使其成为唯一的。

public class MyEntityPk implements Serializable {

   private Integer aId;
   private Integer bId;

   // IMPORTANT: Override equals() and hashCode()
}


@IdClass(MyEntityPk.class)
@Entity
@Table(name = "my_entity")
public class MyEntity implements Serializable {

    @Id
    private Integer aId;
    @Id
    private Integer bId;

    @MapsId("aId")
    @ManyToOne
    private A a;

    @MapsId("bId")
    @OneToOne
    private B b;

请在 Hibernate 文档 https://docs.jboss.org/hibernate/orm/5.4/userguide/html_single/Hibernate_User_Guide.html#identifiers-derived 中查找更多信息