Hibernate 关系映射(一)基于外键关联的N:1

1.单向 N:1 无中间表
例如:一个地方有多个人住.
     每个同学记住老师,比老师记住每个同学简单.
(在 n 的一端进行配置效率高)

N            1
Person   : Address
Student  : Teacher


注意:这样在Person中会多一列 对address_id 的外键引用列

Pesron


Address

public class Address {
	private Integer aid;
	private String addressDetail;

        //set get ...
}

public class Person {

	private Integer pid;
	private String name;
	private int age;
	
	private Address address;  //单向(N:1)  反过来(1:1) 
	//set  get...
}

#主表
DROP TABLE IF EXISTS mytest.address;
CREATE TABLE mytest.address
(
aid         INTEGER NOT NULL,
addressdesc VARCHAR (400),
PRIMARY KEY (aid)
);

#子表
DROP TABLE IF EXISTS mytest.person;

CREATE TABLE mytest.person
(
pid        INTEGER NOT NULL,
name       VARCHAR (50) NOT NULL,
age        TINYINT NOT NULL DEFAULT 0,
address_id INTEGER,
PRIMARY KEY (pid),
KEY address_id (address_id),
CONSTRAINT person_ibfk_1 FOREIGN KEY (address_id) REFERENCES address (aid)
);

   <!--person-->
  <hibernate-mapping package="com.sh.study.model">
	<class name="Person" table="PERSON">
		<id name="pid" type="java.lang.Integer" column="PID">
			<generator class="increment"/>
		</id> 
		<property name="name" type="java.lang.String">
			<column name="NAME" />
		</property>
		<property name="age" type="int">
			<column name="AGE" />
		</property>
		<!--name就是 Person中 的那个 address  
			cascade="all" 
		-->
		<many-to-one name="address" cascade="all"
			class="Address" column="address_id">
		</many-to-one>
	</class>
</hibernate-mapping>


   <!--address-->
  <hibernate-mapping package="com.sh.study.model">
	<class name="Address" table="ADDRESS">
		<id name="aid" type="java.lang.Integer" column="AID">
			<generator class="increment"/>
		</id> 
		<property name="addressdesc" type="java.lang.String">
			<column name="ADDRESSDESC" />
		</property>
	</class>
</hibernate-mapping>

//test

public class TestHibernate {
	private ApplicationContext act;
	private SessionFactory factory;
	@Before
	public void init(){
		act = new ClassPathXmlApplicationContext("classpath:spring/applicationContext.xml");
		factory= act.getBean("sessionFactory",SessionFactory.class);
	}
	//测试 无  中间表的 N:1 关系
	@Test
	public void test1() {
		Session session=factory.getCurrentSession();
		Transaction tx=session.beginTransaction();
		Person p=new Person();
		p.setAge(12);
		p.setName("Yeeku");
		Address address=new Address();
		address.setAddressdesc("北京海淀区");
		p.setAddress(address);

		//持久化对象
		session.persist(p);
		session.flush();
		tx.commit();
	
		//如果不是使用的SessionFactory.getSession()来获得Session。
		//而是使用SessionFactory.getCurrentSession()方法来获得Session时，
		//当事务结束的时候，不管是提交还是回滚事务，hibernate会自动关闭Session的，
		//session.close();
	}
	
	//测试 无  中间表的 N:1 关系
	@Test
	public void test2() {
		Session session=factory.getCurrentSession();
		Transaction tx=session.beginTransaction();
		Person p=new Person();
		p.setAge(12);
		p.setName("Yeeku");
		Address address=new Address();
		address.setAddressdesc("北京海淀区");
		p.setAddress(address);

		//持久化对象
		session.persist(p);
		//修改 Person的 地址 
		Address address1=new Address();
		address1.setAddressdesc("上海虹口");
		p.setAddress(address1);
		tx.commit();
	}
}

注意:cascade="all"

<!--如果Person配置-->
<hibernate-mapping package="com.sh.study.model">
	<class name="Person" table="PERSON">
		<id name="pid" type="java.lang.Integer" column="PID">
			<generator class="increment"/>
		</id> 
		<property name="name" type="java.lang.String">
			<column name="NAME" />
		</property>
		<property name="age" type="int">
			<column name="AGE" />
		</property>
		<!--如果少了 下面这个  
			cascade="all" 
		-->
		<many-to-one name="address" 
			class="Address" column="address_id">
		</many-to-one>
	</class>
</hibernate-mapping>

会出现异常:

org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: com.sh.study.model.Address