【hibernate框架】多对一单向关联(XML实现)

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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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