hibernate之one-to-many详细

1.
拿Classes和Student为例。

package com.test;

import java.util.Set;

public class Classes {

	private Integer classesId;
	private String classesName;
	private Set students;


	public Set getStudents() {
		return students;
	}
	public void setStudents(Set students) {
		this.students = students;
	}
	public Integer getClassesId() {
		return classesId;
	}
	public void setClassesId(Integer classesId) {
		this.classesId = classesId;
	}
	public String getClassesName() {
		return classesName;
	}
	public void setClassesName(String classesName) {
		this.classesName = classesName;
	}

}

<?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.test.Classes" table="classes">
		<id name="classesId" column="classes_id">
			<generator class="native"/>
		</id>
		<property name="classesName" column="classes_name"/>

		<set name="students">
			<!-- key是外键 -->
			<key column="cls_id"/>
			<one-to-many class="com.test.Student"/>	
		</set>
	</class>
</hibernate-mapping>

package com.test;

public class Student {

	private Integer studentId;
	private String studentName;
	public Integer getStudentId() {
		return studentId;
	}
	public void setStudentId(Integer studentId) {
		this.studentId = studentId;
	}
	public String getStudentName() {
		return studentName;
	}
	public void setStudentName(String studentName) {
		this.studentName = studentName;
	}
}

<?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.test.Student" table="student">
		<id name="studentId" column="student_id">
			<generator class="native"/>
		</id>
		<property name="studentName" column="student_name"/>
	</class>
</hibernate-mapping>

测试类：

public  void testAdd(){

		Session session = HibernateUtils.getSession();
		session.beginTransaction();

		Student student1 = new Student();
		student1.setStudentName("zhangsan");

		Student student2 = new Student();
		student2.setStudentName("lisi");

		Classes classes = new Classes();
		classes.setClassesName("02-1");
		Set students = new HashSet();
		students.add(student1);
		students.add(student2);

		classes.setStudents(students);

		session.save(classes);


		session.getTransaction().commit();
		HibernateUtils.closeSession(session);
	}

这时会报org.hibernate.TransientObjectException异常，说明save classes对象时，student1和student2还是游离态对象,但是因为是一对多关系，所以一方Classes要知道多方的信息，这时多方还是游离态，所以就报错了。[color=red]可以设置让 Student对象级联更新[/color]。如下：

<set name="students" cascade="all">
			<!-- key是外键 -->
			<key column="cls_id"/>
			<one-to-many class="com.test.Student"/>	
		</set>

执行的sql代码如下：

Hibernate: insert into classes (classes_name) values (?)
Hibernate: insert into student (student_name) values (?)
Hibernate: insert into student (student_name) values (?)
Hibernate: update student set cls_id=? where student_id=?
Hibernate: update student set cls_id=? where student_id=?

[color=red]可以看出是先insert,再update.这样有个缺点：如果数据量很大时会严重影响效率。可以通过inverse标签反转下。[/color]

	<set name="students" cascade="all" inverse="true">
			<!-- key是外键 -->
			<key column="cls_id"/>
			<one-to-many class="com.test.Student"/>	
		</set>

执行的sql如下：

Hibernate: insert into classes (classes_name) values (?)
Hibernate: insert into student (student_name) values (?)
Hibernate: insert into student (student_name) values (?)

但是这时数据库记录里外键的值是null.[color=red]所以还需要配置双向关联。[/color]

Studnet.hbm.xml里添加:

<many-to-one name="classes" column="cls_id"></many-to-one>

Student.java里添加：

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

测试类里添加：

classes.setStudents(students);
		student1.setClasses(classes);
		student2.setClasses(classes);

[color=red]如果不设置级联更新，也可以这样:[/color]
Classes.hbm.xml

<set name="students" inverse="true" >
			<!-- key是外键 -->
			<key column="cls_id"/>
			<one-to-many class="com.test.Student"/>	
		</set>

Student.hbm.xml不变：

<many-to-one name="classes" column="cls_id"></many-to-one>

测试类：

public  void testAdd1(){

		Session session = HibernateUtils.getSession();
		session.beginTransaction();

		Student student1 = new Student();
		student1.setStudentName("zhangsan");

		Student student2 = new Student();
		student2.setStudentName("lisi");

		Classes classes = new Classes();
		classes.setClassesName("02-1");

		student1.setClasses(classes);
		student2.setClasses(classes);

		session.save(classes);
		session.save(student1);
		session.save(student2);

		session.getTransaction().commit();
		HibernateUtils.closeSession(session);
	}

执行sql:

Hibernate: insert into classes (classes_name) values (?)
Hibernate: insert into student (student_name, cls_id) values (?, ?)
Hibernate: insert into student (student_name, cls_id) values (?, ?)

[color=red]
总结:[/color]

1.都需要设置双向关联（指的是配置文件）
2.都需要设置inverse=true。(对级联来说，不用先insert,再update了)
3.设置了 cascade以后，save的是Classes对象，Student会级联保存。前提是双方关联：
classes.setStudents(students);
student1.setClasses(classes);
student2.setClasses(classes);
4.没有设置cascade的话，save的是Student对象。不过save Student对象以前，要先save Classes对象。