映射：一对一（人与身份）和多对多（老师对学生）

Posted 2020-10-09 古兰精

一、一对一：

第一种情况：IdCard类的ID 既是主键又是外键

package dao.po;  
//人
public class Person  {  
    private int id; //ID
    private String name;//姓名  
    private IdCard idCard; //身份证(对象)  
}  

package dao.po;  
//身份主类 
public class IdCard{  
    private int id;//ID  
    private String address;//地址  
    private Person person; //身份证的主人  
}  

<?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="dao.po">  
    <class name="Person">       
        <id name="id">  
            <generator class="native"/>  
        </id>       
        <property name="name" not-null="true" length="255" column="`name`"/> 
        <!-- 这里做一对一映射    -->  
       <!-- 没什么好说的,就是一 one-to-one -->          
       <one-to-one name="idCard" ></one-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="dao.po">  
    <class name="IdCard" table="id_card">       
        <id name="id">  
            <!-- 注意:  -->  
            <!-- 本类的id 既是主键,又是外键  -->  
          <!-- IdCard对象的是从对象, Person是主对象, 先有主,后有从.  -->           
            <generator class="foreign">  
                <!-- 此处的person是IdCard类的一个属性 -->  
                <param name="property">person</param>  
            </generator>
        </id>       
        <property name="address" not-null="true" length="255" column="`address`"/>
        <!-- 此处的person是IdCard类的一个属性 -->  
        <!-- constrained="true" 对生成的数据表产生约束,id_card表的id既是主键,又是外键 -->    
        <!-- constrained="false" 表结构上没有约束, 取何值对对象关系没影响,只是对表结构有影响-->  
        <one-to-one name="person" constrained="true"/>  
    </class>
</hibernate-mapping>  

　　保存与查询：

package dao;  
import org.hibernate.Hibernate;  
import org.hibernate.Session;  
import org.hibernate.Transaction;  
import dao.po.IdCard;  
import dao.po.Person;  
public class One2One{
    public static void main(final String[] args){  
        add();  
        final IdCard c1 = query(5);  
        System.out.println(c1.getPerson().getName());  
    }
    //保存
    public static void add(){  
        final Person p1 = new Person();  
        p1.setName("小明");
        final IdCard c1 = new IdCard();  
        c1.setAddress("北京市海淀区上地");  
        p1.setIdCard(c1); //建立关联(不能少)  
       c1.setPerson(p1); //建立关联(不能少)  
        Session session = null;  
        try{  
            session = HibernateUtil.getSeesion();  
            final Transaction tx = session.beginTransaction();  
            //session.save(p1); //  
            session.save(c1); //身份证是从对象依赖于主对象, "身份证"依赖于"人",   
           //保存 c1 , 也会自动保存 他依赖的p1,前题是c1 与p1要关联 
            tx.commit();
            //SQL 如下:  
            //Hibernate: insert into Person (`name`) values (?)  
            //Hibernate: insert into id_card (`address`, id) values (?, ?)  
            //虽然只执行了session.save(c1) , 但有两个insert 
        }finally{  
            if (session != null){  
                session.close();  
            }  
        }  
    }  
    //查询 身份证
    public static IdCard query(final int id){  
        Session session = null;  
        try{  
            session = HibernateUtil.getSeesion();  
            final IdCard c1 = (IdCard) session.get(IdCard.class, id);  
            Hibernate.initialize(c1.getPerson());  
            return c1;  
            //SQL 如下:  
            //Hibernate: select idcard0_.id as id3_0_, idcard0_.`address` as address2_3_0_ from id_card idcard0_ where idcard0_.id=?  
            //Hibernate: select person0_.id as id2_1_, person0_.`name` as name2_2_1_, idcard1_.id as id3_0_, idcard1_.`address` as address2_3_0_ from Person person0_ left outer join id_card idcard1_ on person0_.id=idcard1_.id where person0_.id=?  
            //注意person表又连了id-card表  
        }finally{  
            if (session != null){  
                session.close();  
            }  
        }  
    }  
}  

第二种情况：IdCard类的ID 只是主键， IdCard类多对一属性 person_id做 外键，指向Person类的主键。

　　这种情况有点像 多对一，关系如下图：

　　两个配置文件内容如下

 

二、多对多：

　　多对多由于在性能和操作都不大理想，所以使用很少。实际使用中最好转换为一对多的模型。Hibernate会为我们创建中间关联表，转换成两个一对多。

package dao.po;  
import java.util.Set;  
//学生类  
public class Student{  
    private int id;
    private String name;
    private Set<Teacher> teachers;  
}  

package dao.po;  
import java.util.Set;  
//老师类 
public class Teacher{  
    private int id;
    private String name;
    private Set<Student> students;  
}

<?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="dao.po">
    <class name="Student">          
        <id name="id">  
            <generator class="native"/>  
        </id>       
        <property name="name" not-null="true" length="255" column="`name`"/>   
        <!-- name="teachers" 表示:Student类中有一个属性叫teachers (是Set集合)-->  
        <!-- table="teacher_student" 表示:中间表表名叫teacher_student -->           
        <set name="teachers" table="teacher_student">
            <!-- column="student_id" 表示:中间表teacher_student的字段-->  
            <!-- Student类的id与中间表teacher_student的字段student_id对应-->  
            <key column="student_id"/>
            <!-- column="teacher_id" 表示:中间表teacher_student的字段-->  
            <!-- class="Teacher" 表示:中间表teacher_student的字段teacher_id与 Teacher类的id对应-->  
            <many-to-many class="Teacher" column="teacher_id"/>  
        </set>  
    </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="dao.po">
    <class name="Teacher">          
        <id name="id">  
            <generator class="native"/>  
        </id>       
        <property name="name" not-null="true" length="255" column="`name`"/>    
        <set name="students" table="teacher_student">  
            <key column="teacher_id"/>              
            <many-to-many class="Student" column="student_id"/>  
        </set>  
    </class>
</hibernate-mapping> 

　　保存操作：

package dao; 
import java.util.HashSet;  
import java.util.Set;    
import org.hibernate.Session;  
import org.hibernate.Transaction;  
import dao.po.Student;  
import dao.po.Teacher;  
public class Many2Many {  
    public static void main(final String[] args){  
        add();  
    }  
    //添加几个老师与学生 
    public static void add(){  
        final Teacher t1 = new Teacher();  
        t1.setName("语文老师"); 
        final Teacher t2 = new Teacher();  
        t2.setName("数学老师");
        final Student s1 = new Student();  
        s1.setName("学生小明");
        final Student s2 = new Student();  
        s2.setName("学生小红");
        //final Set<Teacher> set_t = new HashSet<Teacher>();  
        //set_t.add(t1);  
        //set_t.add(t2); 
        final Set<Student> set_s = new HashSet<Student>();  
        set_s.add(s1);  
        set_s.add(s2);
        //给老师set 学生  
        t1.setStudents(set_s); //建立关联,只在一方面建立关联就行了, 不可给学生set()了老师,再老师set()了学生  
        t2.setStudents(set_s); //建立关联,只在一方面建立关联就行了, 不可给学生建了关联,再老师建关联
        //给学生set老师,不可以再做
        Session session = null;  
        try{  
            session = HibernateUtil.getSeesion();  
            final Transaction tx = session.beginTransaction();  
            session.save(t1);  
            session.save(t2);  
            session.save(s1);  
            session.save(s2);
            tx.commit();
        }finally{  
            if (session != null){  
                session.close();  
            }  
        }  
    }
}  
// 中间表teacher_student 表结构如下 :  
//| teacher_student | CREATE TABLE `teacher_student` (  
//  `teacher_id` int(11) NOT NULL,  
//  `student_id` int(11) NOT NULL,  
//  PRIMARY KEY  (`student_id`,`teacher_id`),  
//  KEY `FK2E2EF2DE4BF3A147` (`teacher_id`),  
//  KEY `FK2E2EF2DE3B5856A7` (`student_id`),  
//  CONSTRAINT `FK2E2EF2DE3B5856A7` FOREIGN KEY (`student_id`) REFERENCES `student  
//` (`id`),  
//  CONSTRAINT `FK2E2EF2DE4BF3A147` FOREIGN KEY (`teacher_id`) REFERENCES `teacher  
//` (`id`)  
//) ENGINE=InnoDB DEFAULT CHARSET=utf8 |