如何用JPA注释Map<Entity，Entity>？

Question

这个应用程序的栈：Spring MVC，Spring DataJPA，Hibernate。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
}

Tutor

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

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

为了保存student-tutor-theme关系，我希望使用这个表(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
)

如何对实体中的tutors和students字段进行注释，因为它们的内容正确存在于此表中？

Answer 1

正如提到的@kolossus :使用@MapKeyJoinColumn1 1注释，使类(或映射字段)看起来如下(您可以忽略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;

}

既然如此，这样的东西就会被创造出来：

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

1:参见Javadoc documentation of @MapKeyJoinColumn中的其他一些样本