如何使用JPA注释地图<实体，实体>？

Question

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

:

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

:

@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
}

对于保存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
)

如何在实体中注释tutors和students字段，因为它们的内容正确地存在于该表中?

Answer 1

就像@kolossus提到的:使用@MapKeyJoinColumn¹注释，使类(或映射字段)看起来像这样(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;
}

---

在此基础上，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

---

¹:参见@MapKeyJoinColumn的Javadoc文档获取其他示例