三个表之间的@ManyToMany 关系

Question

我的 Spring JPA 应用程序中有三个独立的实体 - 用户、部门、角色

我的数据库中有一个连接表来关联这些实体中的每一个：USER_DEPARTMENT_ROLE

我的问题是，如何在我的实体类中定义这种关系？我是否必须在每个单独的实体中定义 @ManyToMany 关系？我知道如何定义两个表之间的这种关系，但是对于两个以上的表我不知道从哪里开始。

感谢任何帮助！

Answer 1

如果您的联接表中映射了两个以上的关系，那么我建议创建一个单独的实体，用于映射该特定表。

问题是您是否可以有一个不同的 id 列作为人工主键，或者您必须坚持使用由三个外键构建的复合主键。

如果您可以添加人工 id（这是设计数据库的现代方式），那么您的映射应该如下所示：

选项 1

class User {
    @OneToMany(mappedBy = "user", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

class Department{
    @OneToMany(mappedBy = "department", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

class Role{
    @OneToMany(mappedBy = "role", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

class UserDepartmentRoleLink {

    @Id
    @GeneratedValue
    private Long id;

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "department_id")
    private Department department;

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "role_id")
    private Role role;

}

关于为多对多关系设置级联类型是棘手的，对于涉及三个表的多对多来说甚至更棘手，因为每个实体都可以根据情况扮演父母或孩子的角色。我建议只坚持cascade = {CascadeType.PERSIST, CascadeType.MERGE} 并手动处理其他操作。

如果您必须保留复合主键，那么您应该添加额外的 Id 类并将链接实体更改为以下内容：

选项 2

class User {
    @OneToMany(mappedBy = "linkPk.user", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

class Department{
    @OneToMany(mappedBy = "linkPk.department", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

class Role{
    @OneToMany(mappedBy = "linkPk.role", cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    private Set<UserDepartmentRoleLink> userDepartmentRoleLinks;
}

联动表

class UserDepartmentRoleLink {

    @EmbeddedId
    private UserDepartmentRoleLinkId linkPk 
          = new UserDepartmentRoleLinkId(); 

    @Transient
    public User getUser() {
        return getLinkPk().getUser();
    }

    @Transient
    public User getDepartment() {
        return getLinkPk().getDepartment();
    }

    @Transient
    public User getRole() {
        return getLinkPk().getRole();
    }    
 }

@Embeddable
public class UserDepartmentRoleLinkId implements java.io.Serializable {

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "user_id")
    private User user;

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "department_id")
    private Department department;

    @ManyToOne(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinColumn(name = "role_id")
    private Role role;

最重要的是，您可以在此处使用多对多，如本文所述 -> example。但在我看来，如果你像上面那样映射链接表，你会省去很多麻烦。最后，电话是你的..