【hibernate框架】多对一单向关联(XML实现)

一个组有多个用户，一个用户只能属于一个组。
使用XML来实现多对一单项关联

Group.java:

package cn.edu.hpu.many2one;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

public class Group {
	private int id;
	private String name;

	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;
	}

}

User.java:

package cn.edu.hpu.many2one;

import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

public class User {
	private int id;
	private String name;
	private Group group;

	public Group getGroup() {
		return group;
	}
	public void setGroup(Group group) {
		this.group = group;
	}

	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;
	}

}

User.hbm.xml:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="cn.edu.hpu.many2one">
    <class name="User" table="x_user" dynamic-update="true">
    	<!-- 主键 这里的主键名字刚好也是'id'-->
			<id name="id" column="id">
			    <generator class="native"></generator>
			</id>  

        <property name="name"></property>
        <many-to-one name="group" column="groupId"/>
    </class>
</hibernate-mapping>

Group.hbm.xml:

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="cn.edu.hpu.many2one">
    <class name="Group" table="x_group" dynamic-update="true">
    	<!-- 主键 这里的主键名字刚好也是'id'-->
			<id name="id" column="id">
			    <generator class="native"></generator>
			</id>  

        <property name="name"></property> 

    </class>
</hibernate-mapping>

在hibernate.cfg.xml中配置：

<mapping resource="cn/edu/hpu/many2one/Group.hbm.xml"/>
<mapping resource="cn/edu/hpu/many2one/User.hbm.xml"/> 

建表语句：
alter table x_user 
        drop 
        foreign key FKD2372F32E3D521A

    drop table if exists x_group

    drop table if exists x_user

    create table x_group (
        id integer not null auto_increment,
        name varchar(255),
        primary key (id)
    )

    create table x_user (
        id integer not null auto_increment,
        name varchar(255),
        groupId integer,
        primary key (id)
    )

    alter table x_user 
        add index FKD2372F32E3D521A (groupId), 
        add constraint FKD2372F32E3D521A 
        foreign key (groupId) 
        references x_group (id)
schema export complete

结果在x_user表里的groupId属性上加了指向group的id的外键。（如果groupId存的数不是属于x_group表中数据的任何一个id，那么就不会被保存而报错，这就是外键的精髓）

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