• 由“多”方可知“一”方的信息，比如多個員工使用同一棟公寓，員工可以知道公寓的信息，而公寓無法知道員工的信息。

案例一：

• pojo類

 public class Department {    //公寓類，“一”方
  
        private int id;  
        private String name;  
          //setter和getter方法....  
    }  

public class Employee {   //員工類，“多”方 ，既然是多對一，多方知道一放的信息，那麽在多方中必須有一方的對象，
  
    private int
 
 id;  
    private String name;  
    private Department department;  
    //setter和getter方法  
}  

• 實體映射文件

Department.hbm.xml如下：

    <?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.reiyen.hibernate.domain">  
            <class name="Department" >  
                <id name="id" >  
                    <generator class="native" />  
                </id>  
                <property name="name" />  
            </
 
class>  
        </hibernate-mapping>  

Employee.hbm.xml如下：

    <?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.reiyen.hibernate.domain">  
            <class name="Employee">  
                <id name="id">  
                    <generator class="native" />  
                </id>  
                <property name="name" />  
                <!-- name="department" name的值必須與Employee中的屬性名一致. -->  
                <many-to-one name="department" column="depart_id" class="Department" />  
            </class>  
        </hibernate-mapping>  

• <many-to-one >元素建立了department屬性和employee表的外鍵depart_id之間的映射.

• <many-to-one name=”department” column=”depart_id” />這種缺省情況,
• hibernate會默認去department對象中查找主鍵值,因為hibernate默認的是外鍵對應另一個表中的主鍵的,
• 如果想對應department中的其它屬性,如”name”,則可以使用<many-to-one name=”department” column=”depart_id” property-ref=”name”/>也可以使用<many-to-one name=”department” />,這與 <many-to-one name=”depart” column=”depart_id” />的唯一區別就是它的column名也為department了,而不是depart_id.

• name: 設定待映射的持久化類的屬性名,此外為employee類的department屬性.

  column: 設定和持久化類的屬性對應的表的外鍵,此外為employee表的外鍵depart_id.

  class(可選):設定持久化類的屬性的類型,此處設定department的類型是Department類.

  not-null(可選):如果為true,表示department屬性不能為null,該屬性的默認值為false.當為true時,生成的employee表中depart_id外鍵會設置not-null約束,所以當Hibernate保存Employee對象時,會先檢查它的department屬性是否為null,如果為null,則會拋出異常.

（十二）Hibernate中的多表操作（1）：單向多對一