JPA 多对一 @ManyToOne

一个出版者可以发布多本书

@Entity
public class Book {
    @Id
    String isbn;
    String title;
    @ManyToOne // Book 表会增加 publisher_id 与 Publisher 表主键关联
    Publisher publisher;
}
@Entity
public class Publisher {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) // 自增
    Long id;
    String name;
}

结果

[Hibernate] 
    create table Book (
        isbn varchar(255) not null,
        title varchar(255),
        publisher_id bigint,
        primary key (isbn)
    ) engine=InnoDB
[Hibernate] 
    create table Publisher (
        id bigint not null auto_increment,
        name varchar(255),
        primary key (id)
    ) engine=InnoDB
[Hibernate] 
    alter table Book 
       add constraint FKrb2njmkvio5mhe42empuaiphu 
       foreign key (publisher_id) 
       references Publisher (id)

使用 @OneToMany 双向关联

@Entity
public class Book {
    @Id
    String isbn;
    String title;
    @ManyToOne // Book 表会增加 publisher_id 与 Publisher 表关联
    Publisher publisher;
}
@Entity
public class Publisher {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) // 自增
    Long id;
    String name;
    @OneToMany(mappedBy = "publisher")
    Set<Book> books;
}

结果

[Hibernate] 
    create table Book (
        isbn varchar(255) not null,
        title varchar(255),
        publisher_id bigint,
        primary key (isbn)
    ) engine=InnoDB
[Hibernate] 
    create table Publisher (
        id bigint not null auto_increment,
        name varchar(255),
        primary key (id)
    ) engine=InnoDB
[Hibernate] 
    alter table Book 
       add constraint FKrb2njmkvio5mhe42empuaiphu 
       foreign key (publisher_id) 
       references Publisher (id)

使用 @JoinColumn 手动设置关联信息

@Entity
public class Book {
    @Id
    String isbn;
    String title;
    @ManyToOne
    @JoinColumn(name = "publisher_id", referencedColumnName = "id",
            foreignKey = @ForeignKey(name = "PUBLISHER_ID_FK")
    ) // Book 表会增加 publisher_id 字段与 Publisher 表 id(不写默认主键) 关联
    Publisher publisher;
}
@Entity
public class Publisher {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY) // 自增
    Long id;
    String name;
}

结果

[Hibernate] 
    create table Book (
        isbn varchar(255) not null,
        title varchar(255),
        publisher_id bigint,
        primary key (isbn)
    ) engine=InnoDB
[Hibernate] 
    create table Publisher (
        id bigint not null auto_increment,
        name varchar(255),
        primary key (id)
    ) engine=InnoDB
[Hibernate] 
    alter table Book 
       add constraint PUBLISHER_ID_FK 
       foreign key (publisher_id) 
       references Publisher (id)

 

测试

src/main/resources/META-INF/persistence.xml

<persistence xmlns="https://jakarta.ee/xml/ns/persistence"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="https://jakarta.ee/xml/ns/persistence https://jakarta.ee/xml/ns/persistence/persistence_3_0.xsd"
             version="3.0">
    <persistence-unit name="persistenceUnitName">
        <properties><!-- org.hibernate.cfg.JdbcSettings -->
            <property name="jakarta.persistence.jdbc.user" value="root"/>
            <property name="jakarta.persistence.jdbc.password" value="root"/>
            <property name="jakarta.persistence.jdbc.url" value="jdbc:mysql://127.0.0.1:3306/jpa"/>
            <property name="jakarta.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>

            <property name="hibernate.show_sql" value="true"/>
            <property name="hibernate.format_sql" value="true"/>
            <property name="hibernate.highlight_sql" value="true"/>
            <property name="hibernate.hbm2ddl.auto" value="update"/>
        </properties>
    </persistence-unit>
</persistence>

Main

Persistence.createEntityManagerFactory("persistenceUnitName");
// 保存时, 先保存 1 端, 后保存 n 端, 这样不会多出额外的 UPDATE 语句
// 删除时不能直接删除 1 端, 有外键约束
// 查询时使用左外连接获取 n 端和其关联的 1 端, 可使用 @ManyToOne 的 fetch 属性修改加载策略

 

---

https://docs.jboss.org/hibernate/orm/current/introduction/html_single/Hibernate_Introduction.html#many-to-one

https://docs.jboss.org/hibernate/orm/current/userguide/html_single/Hibernate_User_Guide.html#associations-many-to-one

https://jakarta.ee/learn/docs/jakartaee-tutorial/current/persist/persistence-intro/persistence-intro.html#_multiplicity_in_entity_relationships