Hibernate 一对一映射（二）

  上篇介绍了Hibernate一对一映射通过主键关联，这篇介绍一对一映射通过唯一外键关联，还是用Person和IdCard实例说明。

  唯一外键关联其实是多对一的的特例，采用<many-to-one>标签来映射，指定多的一端unique为true，这样就限制了多的一端的多重性为一，就是这样来映射的。

单向：

Person端：

package com.bjpowernode.hibernate;

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 String getName() {
		return name;
	}

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

	public IdCard getIdCard() {
		return idCard;
	}

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

Person.hbm.xml

<?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>
	<class name="com.bjpowernode.hibernate.Person" table="t_person">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<many-to-one name="idCard" unique="true"/>
	</class>
</hibernate-mapping>

IdCard端：

package com.bjpowernode.hibernate;

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;
	}
}

IdCard.hbm.xml

<?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>
	<class name="com.bjpowernode.hibernate.IdCard" table="t_idCard">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="cardNo"/>
	</class>
</hibernate-mapping>

映射出的表结构如下：

双向：

Person端一样，看IdCard端:

package com.bjpowernode.hibernate;

public class IdCard {
	
	private int id;
	
	private String cardNo;
	
	private Person person;

	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;
	}

	public Person getPerson() {
		return person;
	}

	public void setPerson(Person person) {
		this.person = person;
	}
}

IdCard.hbm.xml

<?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>
	<class name="com.bjpowernode.hibernate.IdCard" table="t_idCard">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="cardNo"/>
		<one-to-one name="person" property-ref="idCard"/>      
	</class>
</hibernate-mapping>

  总结：用了两篇博客写了Hibernate一对一映射，一种实现策略是关联主键，另一种是唯一外键关联。这两种策略比较推荐使用第二种。关联主键不须在表中另外添加字段，通过主键相同来维护关系，采用的主键生成策略是“foreign”。局限在想要建立一对一映射的两张表必须主键一样。而相对于这个情况，第二种策略就好多了，它会另外添加一个字段，来维护这个关系。