曾少贤 2017-12-14
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<br />drop table if exists t_employee<br />--建表<br />create table t_department (id integer primary key , name varchar(255));<br />--建序列<br />CREATE SEQUENCE department_sequence<br />INCREMENT BY 1 -- 每次加几个<br />START WITH 1 -- 从1开始计数<br />NOMAXVALUE -- 不设置最大值<br />NOCYCLE -- 一直累加,不循环<br />NOCACHE -- 不建缓冲区<br />--建立触发器<br />create trigger t_department_trig before<br />insert on t_department for each row when (new.id is null)<br />begin<br /> select department_sequence.nextval into:new.id from dual;<br />end;<br />创建表<br />create table t_employee (id integer primary key , name varchar(255), departmentid integer);<br />--建序列<br />CREATE SEQUENCE employee_sequence<br />INCREMENT BY 1 -- 每次加几个<br />START WITH 1 -- 从1开始计数<br />NOMAXVALUE -- 不设置最大值<br />NOCYCLE -- 一直累加,不循环<br />NOCACHE -- 不建缓冲区<br />NOCACHE -- 不建缓冲区<br />--建立触发器<br />create trigger t_employee_trig before<br />insert on t_employee for each row when (new.id is null)<br />begin<br /> select employee_sequence.nextval into:new.id from dual;<br />end;<br />--创建外键关联<br />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();