Hibernate 的事务控制

Person类的hbm配置文件：

<?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 package="com.model">
   <class name="Person" table="persons">
       <id name="id">
         <generator class="native"></generator>
         </id>
       <property name="name"></property>
 </class>

MyClass类的hbm配置文件：

<?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 package="com.model">
   <class name="MyClass" table="classes">
       <id name="id">
         <generator class="native"></generator>
       </id>
       <property name="className"></property>
       <set name="pers" cascade="save-update" >
            <key>
              <column name="class_id" ></column>
            </key>
            <one-to-many class="Person" />
       </set>
   </class>
</hibernate-mapping>

测试类：

SessionFactory sf=new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
        Session session=sf.openSession();
        session.beginTransaction();
        MyClass mc=new MyClass();
        mc.setClassName("lesing");
        Person per1=new Person();
        per1.setName("g");
        Person per2=new Person();
        per2.setName("g");
        Set<Person> pers=new HashSet<Person>();
        pers.add(per1);
        pers.add(per2);
        mc.setPers(pers);
        session.save(mc);

这里不加session.getTransaction().commit()控制台只打印三条sql语句，而没做persons表中class_id的更新

sql语句：

Hibernate: insert into classes (className) values (?)
Hibernate: insert into persons (name, idcard, dept_id) values (?, ?, ?)
Hibernate: insert into persons (name, idcard, dept_id) values (?, ?, ?)
希望有人帮我解释一下