@ManyToMany三个表之间的关系

Question

我的Spring JPA应用程序中有三个独立的实体 - User,Department,Role

我的数据库中有一个连接表来关联每个实体: 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@ManyToMany在每个单独的实体中定义关系？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.

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Answer 1

如果您在连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.

问题是你是否可以拥有一个独特的id列作为人工主键,或者你必须坚持使用三个外键的复合主键构建.

如果您可以添加该人工ID(这是设计数据库的现代方法),那么您的映射应如下所示:

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,并且对于涉及三个表的许多对很多甚至更棘手,因为每个实体可以根据情况扮演父母或孩子的角色..我建议仅坚持cascade = {CascadeType.PERSIST, CascadeType.MERGE}和处理其他操作手动.

如果必须使用复合主键,则应添加其他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,你可以在这里使用多对多,如本文所述 - > 示例.但是在我看来,如果你按照上面的方式映射链接表,你会省去很多麻烦.最后这个电话是你的..