一个工作组(Group)里可以有多个用户(User)，一个User只属于一个Group，这是典型的多对一的关系。在多对一的关系中正确的数据库设计是在多的这方(在这里是User这方)加一个Group的外键。如果数据库设计的与之相反就会产生冗余，请看下面这个例子：

友情提示：这是错误的设计方法：

先写Annotation版本的多对一单向关联：

先建Group类：

package com.hibernate.model;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name="_Group")
public class Group {
private int id;
private String name;
@Id
@GeneratedValue
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;
}

}

package com.hibernate.model;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;


@Entity
@Table(name="_User")
public class User {
private int id;
private String name;
private Group group;
@ManyToOne
public Group getGroup() {
	return group;
}
public void setGroup(Group group) {
	this.group = group;
}
@Id
@GeneratedValue
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;
}

}

<mapping class="com.hibernate.model.User"/> 
<mapping class="com.hibernate.model.Group"/>

package com.hibernate.model;

import static org.junit.Assert.*;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.hibernate.tool.hbm2ddl.SchemaExport;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class ORMappingTest {
	public static SessionFactory sf = null;
	//@BeforeClass
	public static void beforeClass(){
		try{
			sf = new AnnotationConfiguration().configure().buildSessionFactory();
		}
		catch(Exception e) {
			e.printStackTrace();
		}
		finally{	
		}
	}

	@Test
	public void testSchemaExport(){
		new SchemaExport(new AnnotationConfiguration().configure()).create(false, true);
	}
	
	//@AfterClass
	public static void afterClass(){
		sf.close();
	}


}

我把映射文件贴出来：

User.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 package="com.hibernate.model">

    <class name="User" table="_User">
    <id name="id">
    <generator class="native"/>
    </id>
    <property name="name"></property>
    <many-to-one name="group" column="groupId"></many-to-one>
    </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 package="com.hibernate.model">
    <class name="Group" table="_Group">
    <id name="id">
    <generator class="native"/>
    </id>
    <property name="name"></property>
    </class>
</hibernate-mapping>

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hibernate之多对一单向关联