如何使用 JPA 注释 Map<Entity, Entity>？

Question

应用程序堆栈：Spring MVC、Spring DataJPA、Hibernate。共有三个实体：student、tutor、theme。

主题：

@Entity
@Table(name = "themes")
public class Theme {
    // fields omitted
}

学生：

@Entity
@Table(name = "students")
public class Student {

    private Map<Theme, Tutor> tutors;
    // other fields omitted
}

导师：

@Entity
@Table(name = "tutors")
public class Tutor {

    private Map<Theme, Student> students;
    // other fields omitted
}

为了保存student-tutor-theme 关系，我想使用这个表(PostgreSQL)：

CREATE TABLE themes_students_tutors
(
    theme_id   INTEGER NOT NULL,
    student_id INTEGER NOT NULL,
    tutor_id   INTEGER NOT NULL,
    FOREIGN KEY (theme_id) REFERENCES themes (id) ON DELETE CASCADE,
    FOREIGN KEY (student_id) REFERENCES students (id) ON DELETE CASCADE,
    FOREIGN KEY (tutor_id) REFERENCES tutors (id) ON DELETE CASCADE
)

我如何注释实体中的 tutors 和 students 字段，因为它们的内容正确保留在此表中？

Answer 1

就像@kolossus 提到的：使用@MapKeyJoinColumn¹注解，使类（或映射字段）看起来像这样（可以忽略AbstractPersistable 的扩展）：

学生：

public class Student extends AbstractPersistable<Long> {

    @ManyToMany
    @JoinTable(name = "themes_students_tutors", joinColumns = {
            @JoinColumn(name = "student_id", referencedColumnName = "id") }, inverseJoinColumns = {
                    @JoinColumn(name = "tutor_id", referencedColumnName = "id") })
    @MapKeyJoinColumn(name = "theme_id")
    private Map<Theme, Tutor> tutors;

}

导师：

public class Tutor extends AbstractPersistable<Long> {

    @ManyToMany
    @JoinTable(name = "themes_students_tutors", joinColumns = {
            @JoinColumn(name = "tutor_id", referencedColumnName = "id") }, inverseJoinColumns = {
                    @JoinColumn(name = "student_id", referencedColumnName = "id") })
    @MapKeyJoinColumn(name = "theme_id")
    private Map<Theme, Student> students;

}

---

鉴于此，将创建如下内容：

Hibernate: create table students (id bigint not null, primary key (id))
Hibernate: create table themes (id bigint not null, primary key (id))
Hibernate: create table themes_students_tutors (tutor_id bigint not null, student_id bigint not null, theme_id bigint not null, primary key (student_id, theme_id))
Hibernate: create table tutors (id bigint not null, primary key (id))
Hibernate: alter table themes_students_tutors add constraint FKm5l4is34t5gs14p4skkv3aup7 foreign key (student_id) references students
Hibernate: alter table themes_students_tutors add constraint FK8o0mm5ywi0l4hdxi4lgw4dbnu foreign key (theme_id) references themes
Hibernate: alter table themes_students_tutors add constraint FKa0n6jvie0kmk0pmikcuvtepxh foreign key (tutor_id) references tutors

---

¹：请参阅Javadoc documentation of @MapKeyJoinColumn 了解其他示例