一個工作組(Group)裏可以有多個用戶(User),一個User只屬於一個Group,這是典型的多對一的關係。在多對一的關係中正確的數據庫設計是在多的這方(在這裏是User這方)加一個Group的外鍵。如果數據庫設計的與之相反就會產生冗餘,請看下面這個例子:
友情提示:這是錯誤的設計方法:
GroupId |
GroupName |
UserId |
1 |
Group_1 |
1 |
1 |
Group_1 |
2 |
UserId |
UserName |
1 |
moluo |
2 |
xingzhe |
先寫Annotation版本的多對一單向關聯:
先建Group類:
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;
}
}
再建User類:
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;
}
}
在多的這方加group的外鍵,並在get方法上註明:@manyToOne配置文件:
<mapping class="com.hibernate.model.User"/>
<mapping class="com.hibernate.model.Group"/>
測試用例:
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();
}
}
先寫xml版本的多對一單向關聯:Group和User類以及測試用例同上,不再贅述
我把映射文件貼出來:
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>
Group.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="Group" table="_Group">
<id name="id">
<generator class="native"/>
</id>
<property name="name"></property>
</class>
</hibernate-mapping>
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