如何注釋 Map<entity, entity> 與 JPA？</entity,>

Question

該應用程序的堆棧： Spring MVC ， Spring DataJPA ， Hibernate 。 存在三個實體： student 、 tutor 、 theme 。

主題：

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

學生：

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

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

導師：

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

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

為了保存student-tutor-theme關系，我想使用這個表（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
)

我如何注釋實體中的tutors和students字段，因為他們的內容正確保留在此表中？

Answer 1

就像@kolossus 提到的：使用@MapKeyJoinColumn ¹ 注釋，使類（或 map 字段）看起來像這樣（你可以忽略AbstractPersistable的擴展）：

學生：

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;

}

導師：

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;

}

---

鑒於此，將創建如下內容：

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

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¹：有關其他示例，請參閱@MapKeyJoinColumn的 Javadoc 文檔