JPA一对一关联查询

连接查询与嵌套查询是常见的查询问题，获取一个表的内容时也需要获取其外键所引用表的内容。

在JPA中，我们需要先在实体类中指定这种外键关系，通过@JoinColumn注解，并确定对应关系来实现：

@Data
@Entity
@Table(name = "users_detail")
public class AccountDetail {
 
    @Column(name = "id")
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    int id;
 
    @Column(name = "address")
    String address;
 
    @Column(name = "email")
    String email;
 
    @Column(name = "phone")
    String phone;
 
    @Column(name = "real_name")
    String realName;
}

@Data
@Entity
@Table(name = "users")
public class Account {
 
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    @Id
    int id;
 
    @Column(name = "username")
    String username;
 
    @Column(name = "password")
    String password;
 
    @JoinColumn(name = "detail_id")   //指定存储外键的字段名称
    @OneToOne    //声明为一对一关系
    AccountDetail detail;
}

设定完毕后启动项目，Hibernate会自动修改表结构：

Hibernate: alter table users add column detail_id integer
Hibernate: create table users_detail (id integer not null auto_increment, address varchar(255), email varchar(255), phone varchar(255), real_name varchar(255), primary key (id)) engine=InnoDB
Hibernate: alter table users add constraint FK7gb021edkxf3mdv5bs75ni6jd foreign key (detail_id) references users_detail (id)

此时再次查询会直接将关联数据一并添加：

@Test
void pageAccount() {
    repository.findById(1).ifPresent(System.out::println);
}

Hibernate: select account0_.id as id1_0_0_, account0_.detail_id as detail_i4_0_0_, account0_.password as password2_0_0_, account0_.username as username3_0_0_, accountdet1_.id as id1_1_1_, accountdet1_.address as address2_1_1_, accountdet1_.email as email3_1_1_, accountdet1_.phone as phone4_1_1_, accountdet1_.real_name as real_nam5_1_1_ from users account0_ left outer join users_detail accountdet1_ on account0_.detail_id=accountdet1_.id where account0_.id=?
Account(id=1, username=Test, password=123456, detail=AccountDetail(id=1, address=四川省成都市青羊区, email=8371289@qq.com, phone=1234567890, realName=本伟))

懒加载

那要是我们只想要Account的数据，不想要用户详细信息数据怎么办呢？我希望在我要用的时候再获取详细信息，这样可以节省一些网络开销，我们可以设置懒加载，这样只有在需要时才会向数据库获取：

@JoinColumn(name = "detail_id")
@OneToOne(fetch = FetchType.LAZY)    //将获取类型改为LAZY
AccountDetail detail;

接着我们测试一下：

@Transactional   //懒加载属性需要在事务环境下获取，因为repository方法调用完后Session会立即关闭
@Test
void pageAccount() {
    repository.findById(1).ifPresent(account -> {
        System.out.println(account.getUsername());   //获取用户名
        System.out.println(account.getDetail());  //获取详细信息（懒加载）
    });
}

Hibernate: select account0_.id as id1_0_0_, account0_.detail_id as detail_i4_0_0_, account0_.password as password2_0_0_, account0_.username as username3_0_0_ from users account0_ where account0_.id=?
Test
Hibernate: select accountdet0_.id as id1_1_0_, accountdet0_.address as address2_1_0_, accountdet0_.email as email3_1_0_, accountdet0_.phone as phone4_1_0_, accountdet0_.real_name as real_nam5_1_0_ from users_detail accountdet0_ where accountdet0_.id=?
AccountDetail(id=1, address=四川省成都市青羊区, email=8371289@qq.com, phone=1234567890, realName=卢本)

可以看到，获取用户名之前，并没有去查询用户的详细信息，而是当我们获取详细信息时才进行查询并返回AccountDetail对象。