传智播客Hibernate视频教程学习笔记22

 多对多关系的检索：

 

test.java

import java.util.HashSet;import java.util.Iterator;import java.util.Set;import domain.Student;import domain.Teacher;import hibernate.HibernateUtil;public class test {/** * @param args */public static void main(String[] args) {Teacher t1 = new Teacher();t1.setName("t1_name");Teacher t2 = new Teacher();t2.setName("t2_name");Student s1 = new Student();s1.setName("s1_name");Student s2 = new Student();s2.setName("s2_name");Set<Student> ss = new HashSet<Student>();ss.add(s1);ss.add(s2);Set<Teacher> ts = new HashSet<Teacher>();ts.add(t1);ts.add(t2);t1.setStudents(ss);HibernateUtil.add(s1);HibernateUtil.add(s2);HibernateUtil.add(t1);HibernateUtil.add(t2);t1 = (Teacher)HibernateUtil.get(Teacher.class, 1);Iterator<Student> itor = t1.getStudents().iterator();while(itor.hasNext()){System.out.println(itor.next().getName());}}}

和以前一样加上禁用懒加载。

 

Hibernate: select teacher0_.id as id5_0_, teacher0_.name as name5_0_ from Teacher teacher0_ where teacher0_.id=?

根据teacher编号查询teacher信息
Hibernate: select students0_.teacher_id as teacher1_1_, students0_.student_id as student2_1_, student1_.id as id7_0_, student1_.name as name7_0_ from teacher_student students0_ left outer join Student student1_ on students0_.student_id=student1_.id where students0_.teacher_id=?
连接两张表查询teacher中student集合的信息

Hibernate: select teachers0_.student_id as student2_1_, teachers0_.teacher_id as teacher1_1_, teacher1_.id as id5_0_, teacher1_.name as name5_0_ from teacher_student teachers0_ left outer join Teacher teacher1_ on teachers0_.teacher_id=teacher1_.id where teachers0_.student_id=?
连接两张表查询student中teacher集合信息

Hibernate: select teachers0_.student_id as student2_1_, teachers0_.teacher_id as teacher1_1_, teacher1_.id as id5_0_, teacher1_.name as name5_0_ from teacher_student teachers0_ left outer join Teacher teacher1_ on teachers0_.teacher_id=teacher1_.id where teachers0_.student_id=?

连接两张表查询student中teacher集合信息

 

有四条查询语句了。可见这样的查询效率非常第，这一点也是我非常疑惑的，系统里如果这样设计，学生教师关联的信息量肯定是非常大的，这样子的话，程序不死掉才怪。视频上说一般情况下建立多对一关系，而不需要一对多和多对多了，信息量实在太大了，看来设计方式上还是要好好了解一下，一般情况下的系统是如何设计的，才好下手。