8.2 Hibernate：多对一单向关联（unidirectional）

1 建立多对一应用场景

以人与手机的关系为例，同一时间点上，一部手机只能由一个人使用，而一个人可以同时使用一部或多部手机。手机与人之间是多对一的关系。
使用 MySQL 数据库：

CREATE TABLE `test`.`person` (  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,  `name` VARCHAR(16) NOT NULL,  `birth` TIMESTAMP NOT NULL,  PRIMARY KEY (`id`),  UNIQUE INDEX `id_UNIQUE` (`id` ASC))ENGINE = InnoDBDEFAULT CHARACTER SET = utf8COMMENT = '人';

CREATE TABLE `test`.`phone` (  `id` INT UNSIGNED NOT NULL AUTO_INCREMENT,  `imei` VARCHAR(15) NOT NULL,  `number` VARCHAR(11) NOT NULL,  `owner_id` INT UNSIGNED NULL,  PRIMARY KEY (`id`),  UNIQUE INDEX `id_UNIQUE` (`id` ASC),  UNIQUE INDEX `imei_UNIQUE` (`imei` ASC),  UNIQUE INDEX `number_UNIQUE` (`number` ASC),  INDEX `FK_PHONE_idx` (`owner_id` ASC),  CONSTRAINT `FK_PHONE`    FOREIGN KEY (`owner_id`)    REFERENCES `test`.`person` (`id`)    ON DELETE NO ACTION    ON UPDATE NO ACTION)ENGINE = InnoDBDEFAULT CHARACTER SET = utf8COMMENT = '手机';

2 定义映射类
2.1 表 person 的映射类定义：

package hibernate;import java.util.Date;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.Table;@Entity@Table(name = "person")public class Person {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    @Column(name = "id")    private int id;    @Column(name = "name")    private String name;    @Column(name = "birth")    private Date birth;    public Person() {}    public Person(String name, Date birth) {        this.name = name;        this.birth = birth;    }    // 省略 Getters 和 Setters ...}

2.2 表 phone 的映射类定义：

package hibernate;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.ForeignKey;import javax.persistence.GeneratedValue;import javax.persistence.GenerationType;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.ManyToOne;import javax.persistence.Table;@Entity@Table(name = "phone")public class Phone {    @Id    @GeneratedValue(strategy = GenerationType.IDENTITY)    @Column(name = "id")    private int id;    @Column(name = "imei")    private String imei;    @Column(name = "number")    private String number;    @ManyToOne    @JoinColumn(name = "owner_id",         foreignKey = @ForeignKey(name = "FK_PHONE"))    private Person person;    public Phone() {}    public Phone(String imei, String number) {        this.imei = imei;        this.number = number;    }    // 省略 Getters 和 Setters ...}

3 测试
3.1 新增

@Testpublic void test() {    Configuration configuration = new Configuration().configure("hibernate.cfg.xml");    SessionFactory sessionFactory = configuration.buildSessionFactory();    Session session = sessionFactory.openSession();    Transaction transaction = session.beginTransaction();    Person person = new Person("Jack", new Date());    session.save(person);    Phone phone1 = new Phone("866661234567890", "18012345678");    phone1.setPerson(person);    session.save(phone1);    Phone phone2 = new Phone("866660987654321", "18087654321");    phone2.setPerson(person);    session.save(phone2);    transaction.commit();    session.close();    sessionFactory.close();}

运行测试，对象 person，phone1 和 phone2 都被持久化存储到对应的 person 和 phone 数据表中，且 phone1 和 phone2 对应记录的外键都是新插入的 person 对应记录的主键（日志省略）。

如果将单元测试代码中 session.save(phone1); 和 session.save(phone2); 屏蔽掉，则只会持久化存储 person 对象。

如果将单元测试代码中 session.save(person); 屏蔽掉，默认情况下单元测试报异常，不能成功持久化存储任一对象数据。

java.lang.IllegalStateException: org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance beforeQuery flushing

由此可见：新增多对一关系数据时，默认首先需要保存“一”方的数据，只能成功保存了一”方的数据后才有可能保存“多”方的数据，除非修改 @ManyToOne 的 cascade 属性，才能实现级联插入和更新（后续会详细阐述 cascade 属性的用途）

3.2 删除
基于以上测试新增的数据进行删除测试。

@Testpublic void test() {    Configuration configuration = new Configuration().configure("hibernate.cfg.xml");    SessionFactory sessionFactory = configuration.buildSessionFactory();    Session session = sessionFactory.openSession();    Transaction transaction = session.beginTransaction();    List<Phone> phones = session.createQuery(" FROM Phone").list();    for (Phone phone : phones) {         session.delete(phone);    }    Person person = (Person) session.createQuery(" FROM Person").list().get(0);    session.delete(person);    transaction.commit();    session.close();    sessionFactory.close();}

注意：
(1) 可以只删除 phone，默认不会对其关联的 person 产生影响，除非修改 @ManyToOne 的 cascade 属性（后续会详细阐述 cascade 属性的用途）；
(2) 删除 person 前必须首先删除外键关联的 phone，否则出现以下异常。

Caused by: com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Cannot delete or update a parent row: a foreign key constraint fails

3.3 更新

@Testpublic void test() {    Configuration configuration = new Configuration().configure("hibernate.cfg.xml");    SessionFactory sessionFactory = configuration.buildSessionFactory();    Session session = sessionFactory.openSession();    Transaction transaction = session.beginTransaction();    Phone phone = (Phone) session.createQuery(" FROM Phone").list().get(0);    phone.setImei("123456789012345");    phone.setNumber("15912345678");    phone.getPerson().setName("Bruce");    phone.getPerson().setBirth(new Date());    transaction.commit();    session.close();    sessionFactory.close();}

运行测试，phone 关联的 person 级联更新。

3.4 查询

@Testpublic void test() {    Configuration configuration = new Configuration().configure("hibernate.cfg.xml");    SessionFactory sessionFactory = configuration.buildSessionFactory();    Session session = sessionFactory.openSession();    Transaction transaction = session.beginTransaction();    List<Phone> phones = session.createQuery(" FROM Phone").list();    for (int i = 1; i <= phones.size(); i++) {        System.out.println("---------- Phone " + i + "----------");        System.out.println("IMEI : " + phones.get(i - 1).getImei());        System.out.println("Number : " + phones.get(i - 1).getNumber());        System.out.println("Owner Name : " + phones.get(i - 1).getPerson().getName());        System.out.println("Owner Birth : " + phones.get(i - 1).getPerson().getBirth());    }    transaction.commit();    session.close();    sessionFactory.close();}

运行测试，打印结果：

---------- Phone 1----------IMEI : 866661234567890Number : 18012345678Owner Name : JackOwner Birth : 2017-07-09 21:07:49.0---------- Phone 2----------IMEI : 866660987654321Number : 18087654321Owner Name : JackOwner Birth : 2017-07-09 21:07:49.0