--------------Hibernate学习 多对一映射 和 一对多映射

Posted 2020-10-17 熬过今夜还有明天

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();