nhibernate中Many-to-Many中间表的问题

代码目的是要实现Course与Student的多对多。问题是多对多的两端插入正常，但中间表中的数据无法插入。小弟才疏学浅，还请各位指教。下面是一些代码：Course类    public class Course
    {
        private string id;
        private string name;
        private Iesi.Collections.ISet students = new Iesi.Collections.HashedSet();        public virtual string Id
        {
            get { return id; }
            set { id = value; }
        }
        public virtual string Name
        {
            get { return name; }
            set { name = value; }
        }
        public virtual Iesi.Collections.ISet Students
        {
            get { return students; }
            set { students = value; }
        }
    }
Course.hbm.xml<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" namespace = "NHibernateDemo" assembly = "NHibernateDemo">
  <class name = "Course" table = "Course">
    <id name = "Id">
      <column name = "CourseId" sql-type = "char(32)" not-null = "true"/>
      <generator class="uuid.hex"/>
    </id>
    <property name = "Name">
      <column name = "Name" length = "16" not-null = "true"/>
    </property>
    <set name = "Students" cascade="all" inverse="true" lazy="false" table="Course_Student">
      <key column="CourseId"/>
      <many-to-many class="Student" column="StudentId"/>
    </set>
  </class>
</hibernate-mapping>
Student类    public class Student
    {
        private string id;
        private string name;
        private Iesi.Collections.ISet courses = new Iesi.Collections.HashedSet();        public virtual string Id
        {
            get { return id; }
            set { id = value; }
        }
        public virtual string Name
        {
            get { return name; }
            set { name = value; }
        }
        public virtual Iesi.Collections.ISet Courses
        {
            get { return courses; }
            set { courses = value; }
        }
    }
Student.hbm.xml<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns = "urn:nhibernate-mapping-2.2" namespace = "NHibernateDemo" assembly = "NHibernateDemo">
  <class name = "Student" table = "Student">
    <id name = "Id">
      <column name = "StudentId" sql-type = "char(32)" not-null = "true"/>
      <generator class="uuid.hex"/>
    </id>
    <property name = "Name">
      <column name = "Name" length = "16" not-null = "true"/>
    </property>
    <set name = "Courses" cascade="all" inverse="true" lazy="false" table="Course_Student">
      <key column="StudentId"/>
      <many-to-many class="Course" column="CourseId"/>
    </set>
  </class>
</hibernate-mapping>
写入数据库的代码            ISession session = NHibernateHelper.GetCurrentSession();            ITransaction tx = session.BeginTransaction();            Course courseOne = new Course();
            courseOne.Name = "Math";            Course courseTwo = new Course();
            courseTwo.Name = "Chinese";            Student studentOne = new Student();
            studentOne.Name = "ZhangThree";            Student studentTwo = new Student();
            studentTwo.Name = "LiFour";            studentOne.Courses.Add(courseOne);
            studentOne.Courses.Add(courseTwo);
            studentTwo.Courses.Add(courseTwo);            courseOne.Students.Add(studentOne);
            courseTwo.Students.Add(studentOne);
            courseTwo.Students.Add(studentTwo);            session.Save(courseOne);
            session.Save(courseTwo);
            session.Save(studentOne);
            session.Save(studentTwo);            tx.Commit();            NHibernateHelper.CloseSession();