现实中有很多场景需要用到多对一或者一对多，比如上面这两个类图所展现出来的，一般情况下，一个部门会有多名员工，一名员工只在一个部门任职。
多对一关联映射
在上面的场景中，对于Employee来说，它跟Department的关系就是多对一。
先写实体类
Employee.java
package entity;public class Employee { ????public int id; ???????public String name; ???????public Department department; ???????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; ???????} ???????public Department getDepartment() { ???????????return department; ???????} ???????public void setDepartment(Department department) { ???????????this.department = department; ???????}}Department.java
package entity;public class Department { ???public int id; ???public 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; ???}}配置多对一关系时，设计实体类时，除了写出最基本的属性（比如Employee的id、name），在对应“多”的那个类（比如Employee.java）中添加对应“一”那个类的引用（比如上面的department）。
映射文件
Employee.hbm.xml
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" ??????????????????????????????????"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><hibernate-mapping> ??<class name="entity.Employee" table="t_employee"> ????<id name="id"> ????????<generator class="native"/> ????</id> ????<property name="name"/> ????<many-to-one name="department" column="departmentid"/> ??</class></hibernate-mapping> ????
Department.hbm.xml
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" ??????????????????????????????????"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><<hibernate-mapping> ????<class name="entity.Department" ?table="t_Department"> ???????<id name="id"> ??????????<generator class="native"/> ???????</id> ???????<property name="name"></property> ????</class></hibernate-mapping> ??????????????????????????????????
映射文件中的内容基本上跟它关联的类中的字段都是对应的。主键配置在<id></id>中，基本字段配置在<property/>中，对其他类的引用配置在<many-to-one/>中。
建表
drop table t_department
drop table if exists t_employee
--建表
create table t_department (id integer primary key , name varchar(255));
--建序列
CREATE SEQUENCE department_sequence
INCREMENT BY 1 -- 每次加几个
START WITH 1 -- 从1开始计数
NOMAXVALUE -- 不设置最大值
NOCYCLE -- 一直累加，不循环
NOCACHE -- 不建缓冲区
--建立触发器
create trigger t_department_trig before
insert on t_department for each row when (new.id is null)
begin
select department_sequence.nextval into:new.id from dual;
end;
创建表
create table t_employee (id integer primary key , name varchar(255), departmentid integer);
--建序列
CREATE SEQUENCE employee_sequence
INCREMENT BY 1 -- 每次加几个
START WITH 1 -- 从1开始计数
NOMAXVALUE -- 不设置最大值
NOCYCLE -- 一直累加，不循环
NOCACHE -- 不建缓冲区
NOCACHE -- 不建缓冲区
--建立触发器
create trigger t_employee_trig before
insert on t_employee for each row when (new.id is null)
begin
select employee_sequence.nextval into:new.id from dual;
end;
--创建外键关联
alter table t_employee add constraint fk_departmentid foreign key (departmentid) references t_department(id);
一执行,发现报错了:
org.hibernate.TransientObjectException: object references an unsaved transient instance - save the transient instance before flushing: entity.Department
一看错误就知道，这是因为department还在Transient状态时，session是不能对其操作的。所以可以在事务提交之前先save一下department：
session.beginTransaction();Department department=new Department();department.setName("信息部");Employee employee1=new Employee();employee1.setName("小胡");employee1.setDepartment(department);Employee employee2=new Employee();employee2.setName("小玉");employee2.setDepartment(department);session.save(department); ??????????session.save(employee1);session.save(employee2);session.getTransaction().commit();--------------Hibernate学习(四) ?多对一映射 和 一对多映射
原文地址：http://www.cnblogs.com/daweige/p/8039246.html