J2EE之路（七）hibernate（03）关联映射
关联映射
多对一(Employee - Department)一对多(Department - Employee)一对一(person - idcard)多对多(teacher - student)组合映射(User - Name)集合映射(set,list,map,sag)inverse和cascade(Employ - department)多对一映射(Employee = Department)
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映射文件<many-to-one name="depart" column="depart_id"/>
可以看出，我们可以只关心对象模型，关系模型由hibernte来维护。
对应一下Department类
package domain;public class Employee {private int id;private String name;private Department depart;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 getDepart() {return depart;}public void setDepart(Department depart) {this.depart = depart;}}?可以看出Employee类持有department类的引用，在配置文件中只要把属性放进名字即可。
看一下自动生生成的Employee表的建表语句，果然生成了外键。
PS：只是标签从property编程了many-to-one其他的貌似都没变
测试一下：
package test;import org.hibernate.Session;import org.hibernate.Transaction;import Util.HibernateUtils;import domain.Department;import domain.Employee;import domain.User;public final class TestMany2One {public static void main(String[] args) {add();query(1);}private static void add() {Session session = null;Transaction transaction = null;try{session = HibernateUtils.getSession();transaction = session.beginTransaction();Department depart = new Department();depart.setName("depart01");Employee employee = new Employee();employee.setName("employee01");employee.setDepart(depart);//对象模型：建立两个对象的关联session.save(depart);session.save(employee);transaction.commit();}catch(Exception e){e.printStackTrace();transaction.rollback();}finally{if(session != null)session.close();}}private static Employee query(int id) {Session session = null;try{session = HibernateUtils.getSession();Employee employee = (Employee) session.get(Employee.class,id);System.out.println(employee.getDepart().getName());return employee;}finally{if(session != null)session.close();}}}?
?一对多(Department - Employee)
?在Department类中增加属性Set<Employee> emps 表示一对多
在Deppartment的配置文件中增加
<set name="emps">
?<key column="depart_id"/>
<one-to-many calss="Employee"/>
</set>
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一对一(person - idcard)
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基于主键的one-to-one(person的映射文件)
<id name="id">
?<generator constrained="true">
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多对多(teacher - student)
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在操作和性能方面都不太理想，所以多对多映射使用较少，实际使用中最好转换成一对多的模型，Hibernate会为我们创建中间关联表，转换成两个一对多。
<set name="teacher" table="teacher_student">
<key column="teacher_id">
<many-to-many column="student">
</>
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组合映射(User - Name)
感觉就是这个对象不是很大，不用使用一张表来表示他比如姓和名直接放进User表就好
标签如下：
<compoment name="name">
<property name="firstName" column="first_name">
<property name="lastName" column="last_name">
</compoment>
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最后零散笔记吧
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级联操作:<set name="emps" cascade="save-update">
一对多，多对一是相互的，一般一的这一方面放弃对关系的维护：<set name="emps" inverse="true">
以上只能是set，list不能指定inverse，所以一对多最好不用List，而用set