一对一关联

两个表通过主键来关联

就是一个表的主键依赖另一个表的主键，如果一个表里面没有数据，那么在另一关联他的表插入数据会报错。

如下：

配置文件：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.lwf.hibernate.pojo" >
	<class name="Person">
		<id name="id">
			<generator class="foreign">
				<param name="property">idCard</param>
			</generator>
		</id>
		<property name="name"/>
		<one-to-one name="idCard" class="IdCard" constrained="true"></one-to-one>
	</class>

	<class name="IdCard">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="cardNo"/>
	</class>
</hibernate-mapping>

 

 

POJO

package com.lwf.hibernate.pojo;

public class Person {

	private int id;

	private String name;
	private IdCard idCard;

	public int getId() {
		return id;
	}

	public void setId(int id) {
		this.id = id;
	}

	public IdCard getIdCard() {
		return idCard;
	}

	public void setIdCard(IdCard idCard) {
		this.idCard = idCard;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}
}

 

 

package com.lwf.hibernate.pojo;

public class IdCard {

	private int id;
	private String cardNo;
	
	
	public int getId() {
		return id;
	}
	public void setId(int id) {
		this.id = id;
	}
	public String getCardNo() {
		return cardNo;
	}
	public void setCardNo(String cardNo) {
		this.cardNo = cardNo;
	}
	
}

 

 

生成的Sql语句：

 create table IdCard (
        id integer not null auto_increment,
        cardNo varchar(255),
        primary key (id)
    )

    create table Person (
        id integer not null,
        name varchar(255),
        primary key (id)
    )
alter table Person 
        add index FK8E488775694BC674 (id), 
        add constraint FK8E488775694BC674 
        foreign key (id) 
        references IdCard (id)

 

 

在数据库中产生两个表：

person表

+-------+--------------+------+-----+---------+-------+
| Field  | Type           | Null | Key | Default | Extra |
+-------+--------------+------+-----+---------+-------+
| id      | int(11)         | NO   | PRI | NULL    |       |
| name | varchar(255) | YES  |     | NULL    |       |
+-------+--------------+------+-----+---------+-------+

 

idcard表

+--------+--------------+------+-----+---------+----------------+
| Field  | Type         | Null | Key | Default | Extra          |
+--------+--------------+------+-----+---------+----------------+
| id     | int(11)      | NO   | PRI | NULL    | auto_increment |
| cardNo | varchar(255) | YES  |     | NULL    |                |
+--------+--------------+------+-----+---------+----------------+

 

因为Person的主键关联于idcard的主键，所以必须先在idcard表中插入数据如

 insert into idcard(cardNo) values('sfsddf');

产生数据；

+----+--------+
| id | cardNo |
+----+--------+
|  1 | sfsddf |
+----+--------+

下面再执行插入数据到person中。

insert into idcard(cardNo) values('sfsddf');

+----+------+
| id | name |
+----+------+
|  1 | dddl |
+----+------+

如果：

 insert into person(id,name) values(3,'dddl');

ERROR 1452 (23000): Cannot add or update a child row: a foreign key constraint f。。
ails (`hibernate`.`person`, CONSTRAINT `FK8E488775694BC674` FOREIGN KEY (`id`) R
EFERENCES `idcard` (`id`))

 

 

在使用hibernate进行插入时只需要

public static void main(String[] args) {
		Session session = openSession();
		
		IdCard idCard = new IdCard();
		idCard.setCardNo("1121212");
		
		Person person = new Person();
		person.setIdCard(idCard);
		person.setName("zhangshang");
		session.save(person);
		
		commitSession(session);
		closeSession(session);
		
	}

生成语句：

Hibernate: 
    /* insert com.lwf.hibernate.pojo.IdCard
        */ insert 
        into
            IdCard
            (cardNo) 
        values
            (?)
Hibernate: 
    /* insert com.lwf.hibernate.pojo.Person
        */ insert 
        into
            Person
            (name, id) 
        values
            (?, ?)

  

 

注意上面我们没有先sesssion.save(idCard);

在一对一关联中会自动先执行被关联对象的插入操作。这说明：

一对一关联默认了级联属性。

 

注意如果上面代码中把person.setIdCard(idCard);注释，那么会报NullPointerException

因为Person依赖IdCard为主键的，所以必须设置idCard属性

 

保存完之后加载

Person person = (Person)session.load(Person.class, 1);
		System.out.println(person.getName());
		System.out.println(person.getIdCard().getCardNo());

生成 语句

Hibernate: 
    /* load com.lwf.hibernate.pojo.Person */ select
        person0_.id as id3_0_,
        person0_.name as name3_0_ 
    from
        Person person0_ 
    where
        person0_.id=?
zhangshang
Hibernate: 
    /* load com.lwf.hibernate.pojo.IdCard */ select
        idcard0_.id as id4_0_,
        idcard0_.cardNo as cardNo4_0_ 
    from
        IdCard idcard0_ 
    where
        idcard0_.id=?
1121212

 

 

删除：

Person person = (Person)session.load(Person.class, 1);
		session.delete(person);

 

将删除person表中id为1的列，删除不会删除与之关联的idCard表的的列。

如果：

IdCard idCard = (IdCard)session.load(IdCard.class, 3);
  session.delete(idCard);

因为有Person表中的主键与之关联，所以删除报错，必须先删除Person表中对应的列。

 

 

双向关联：

上面我们加载的时候都是从person端加载，得到idCard

Person person = (Person)session.load(Person.class, 1);
		System.out.println(person.getName());
		System.out.println(person.getIdCard().getCardNo());

 

那么能不能从idCard端加载，得到person呢

试一下：

IdCard idCard = (IdCard)session.load(IdCard.class,1);
		System.out.println(idCard.getPerson().getName());

 

输出结果：

报错

Hibernate: 
    /* load com.lwf.hibernate.pojo.IdCard */ select
        idcard0_.id as id4_0_,
        idcard0_.cardNo as cardNo4_0_ 
    from
        IdCard idcard0_ 
    where
        idcard0_.id=?
Exception in thread "main" java.lang.NullPointerException
	at com.lwf.hibernate.test.TestPerson.main(TestPerson.java:19)

 

这里我们需要实现一对一的双向关联：

我们只要在idcard的映射文件中加入one-to-one 即可。

如：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.lwf.hibernate.pojo" >
	<class name="Person">
		<id name="id">
			<generator class="foreign">
				<param name="property">idCard</param>
			</generator>
		</id>
		<property name="name"/>
		<one-to-one name="idCard" class="IdCard" constrained="true"></one-to-one>
	</class>

	<class name="IdCard">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="cardNo"/>
		 <one-to-one name="person"></one-to-one>
	</class>
</hibernate-mapping>

 

上面单向与双向双联我们只增加了：

 <one-to-one name="person"></one-to-one>

对应person在IdCard Pojo类中定义属性：

 private Person person; 

并实现get与Set方法即可。

 

 

另一种一对一关联的方法是在配置文件中使用many-to-one，然后使用唯一属性

<many-to-one name="idCard" class="IdCard" column="idCardId" unique="true"></many-to-one>

 

这也就唯一外键关联，在家里即person表中的主键有id和idCardId两个字段，但idCardId关联idCard表的id字段，并且在person表中的idCardId是唯一的。

 

如我们把上例的配置文件改成：

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="com.lwf.hibernate.pojo" >
	<class name="Person">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
	<many-to-one name="idCard" class="IdCard" column="idCardId" unique="true"></many-to-one> 
	</class>

	<class name="IdCard">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="cardNo"/>
			
	</class>
</hibernate-mapping>

对应的表结构：

 

mysql> desc person;
+----------+--------------+------+-----+---------+-------+
| Field    | Type         | Null | Key | Default | Extra |
+----------+--------------+------+-----+---------+-------+
| id       | int(11)      | NO   | PRI | NULL    |       |
| name     | varchar(255) | YES  |     | NULL    |       |
| idCardId | int(11)      | YES  | UNI | NULL    |       |
+----------+--------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> desc idcard;
+--------+--------------+------+-----+---------+----------------+
| Field  | Type         | Null | Key | Default | Extra          |
+--------+--------------+------+-----+---------+----------------+
| id     | int(11)      | NO   | PRI | NULL    | auto_increment |
| cardNo | varchar(255) | YES  |     | NULL    |                |
+--------+--------------+------+-----+---------+----------------+
2 rows in set (0.02 sec)

 

保存

public static void save(Session session){
		IdCard idCard = new IdCard();
		idCard.setCardNo("1121212");
	session.save(idCard);
		Person person = new Person();
		person.setIdCard(idCard);
		person.setName("zhangshang");
		session.save(person);
	}

 

注意在使用many-to-one的时候默认级联属性为false，所以上面要先使用session.save(idCard);
再使用session.save(person);保存。

而上面的one-to-one的时候默认级联属性为true，所以不需要先session.save(idCard);

 保存之后数据表如下：

mysql> select * from person;
+----+------------+----------+
| id | name       | idCardId |
+----+------------+----------+
|  1 | zhangshang |        1 |
+----+------------+----------+
1 row in set (0.00 sec)

mysql> select * from idcard;
+----+---------+
| id | cardNo  |
+----+---------+
|  1 | 1121212 |
+----+---------+
1 row in set (0.00 sec)

 

双向关联:

只要在idcard对应的配置中加入:

<one-to-one name="person" class="Person" property-ref="idCard"></one-to-one>

 

修改后的配置文件如下:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
	"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
	<hibernate-mapping package="com.lwf.hibernate.map">
		<class name="Person">
			<id name="id">
				<generator class="native"/>
			</id>
			<many-to-one name="idCard" column="idCardId" unique="true"/>
			<property name="name"/>
		</class>
		
		<class name="IdCard">
			<id name="id">
				<generator class="native"/>
			</id>
			<property name="cardNo"/>
			<one-to-one name="person" class="Person" property-ref="idCard"></one-to-one>
		</class>
	</hibernate-mapping>

 

测试从person取得idCard的数据:

Person person = (Person)session.load(Person.class, 2);
		System.out.println(person.getName());
		System.out.println(person.getIdCard().getCardNo());
		

 

测试双向,从idCard取得person的数据:

IdCard idCard = (IdCard)session.load(IdCard.class, 1);
		System.out.println(idCard.getCardNo());
		System.out.println(idCard.getPerson().getName());