Hibernate HQL示例八:查询对象导航及Implicit Join

通过Student对象导航到class对象的id

如下：

POJO

package com.bjsxt.hibernate;

import java.util.Date;

public class Student {
	
	private int id;
	
	private String name;
	
    private Date createTime;	
	
	private Classes classes;
	
	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 Classes getClasses() {
		return classes;
	}

	public void setClasses(Classes classes) {
		this.classes = classes;
	}

	public Date getCreateTime() {
		return createTime;
	}

	public void setCreateTime(Date createTime) {
		this.createTime = createTime;
	}
}

 

package com.bjsxt.hibernate;

import java.util.Set;

public class Classes {
	
	private int id;
	
	private String name;
	
	private Set students;
	
	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 Set getStudents() {
		return students;
	}

	public void setStudents(Set students) {
		this.students = students;
	}
	
	
}

 

<?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" cascade="all" order-by="id">
		 -->
		 <set name="students" inverse="true">
			<key column="classid"/>
			<one-to-many class="com.bjsxt.hibernate.Student"/>
		</set>
	</class>
</hibernate-mapping>

 

<?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"/>
		<property name="createTime"/>
		<many-to-one name="classes" column="classid"/>
		<filter name="idfilter" condition="id &lt; :myid"/>
	</class>
	<query name="searchStudents"><![CDATA[
		select s from Student s where s.id < ?
	]]>
	</query>
	<filter-def name="idfilter">
		<filter-param name="myid" type="integer"/>
	</filter-def>
</hibernate-mapping>

 

下面测试

package com.bjsxt.hibernate;

import java.util.Iterator;
import java.util.List;

import org.hibernate.Session;

import junit.framework.TestCase;

/**
 * 对象导航查询测试
 * @author Administrator
 *
 */
public class ObjectNavQueryTest extends TestCase {

	public void testQuery1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			
			List students = session.createQuery("select s.name from Student s where s.classes.id < 2").list();
			for (Iterator iter = students.iterator();iter.hasNext();) {
				String name = (String)iter.next();	
				System.out.println(name);
			}
		}catch(Exception e) {
			e.printStackTrace();
		}finally {
			HibernateUtils.closeSession(session);
		}	
	}	
}

 

上面select s.name from Student s where s.classes.id < 2实际上使用了隐式的inner join

对应的sql语句有两种形式

mysql> select s.name from t_student s ,t_classes c where s.classid = c.id and c.
id<2;

mysql> select t.name from t_student t inner join t_classes c on t.classid=c.id w
here c.id<2;

 

对象导航其实就是一种隐式join的应用。