hibernate学习4之one-to-many单向关联（不推荐）

一对多和多对一的映射策略是一样的，只是站的角度不同.

多对一关联映射：在多的一端加入一个外键指向一的一端，它维护的关系是多指向一
一对多关联映射：在多的一端加入一个外键指向一的一端，它维护的关系是一指向多

先上配置文件：

<!DOCTYPE hibernate-configuration PUBLIC
	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="hibernate.connection.url">jdbc:mysql://localhost/hibernate_one2many_1</property>
		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="hibernate.connection.username">root</property>
		<property name="hibernate.connection.password">bjsxt</property>
		<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
		<property name="hibernate.show_sql">true</property>
		
		<mapping resource="com/bjsxt/hibernate/Classes.hbm.xml"/>
		<mapping resource="com/bjsxt/hibernate/Student.hbm.xml"/>
	</session-factory>
</hibernate-configuration>

public class Student {
	private int id;
	private String name;
	//setter，getter
}

import java.util.Set;

public class Classes {
	private int id;
	private String name;
	private Set students; //一对多，一个班级对应很多学生
	//setter，getter
	
}

Student.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.bjsxt.hibernate.Student" table="t_student">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
	</class>
</hibernate-mapping>

Classes.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.bjsxt.hibernate.Classes" table="t_classes">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<set name="students">
			<key column="classesid"/>
			<one-to-many class="com.bjsxt.hibernate.Student"/>
		</set>
	</class>
</hibernate-mapping>

上面这个配置在Student一端加入了一个classesid外键；也说明set里存放的是Student对象

测试一下：

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import junit.framework.TestCase;

import org.hibernate.Session;

public class One2ManyTest extends TestCase {

	public void testSave1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Student student1 = new Student();
			student1.setName("10");
			session.save(student1);//先save
			
			Student student2 = new Student();
			student2.setName("祖儿");
			session.save(student2);//先save
			
			Set students = new HashSet();
			students.add(student1);
			students.add(student2);
			
			Classes classes = new Classes();
			classes.setName("尚学堂");
			classes.setStudents(students);
			
			//可以正确保存
			session.save(classes);
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Classes classes = (Classes)session.load(Classes.class, 1);
			System.out.println("classes.name=" + classes.getName());
			Set students = classes.getStudents();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println("student.name=" + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}

在一一端维护关系的缺点：
* 如果将t_student表里的classesid字段设置为非空，则无法保存
* 因为不是在student这一端维护关系，所以student不知道是哪个班的，
  所以需要发出多余的update语句来更新关系

因为一对多（单向关联）会发出额外的sql语句，效率没有多对一高，所以通常情况下不采用一对多的单向关联，采用双向关联比较妥当！