Hibernate多对多查询

一个老师教许多学生,一个学生被许多老师教,一个学生有好多书,同一种书被许多同学拥有.
要查询教拥有书"a"的学生的老师!
HQL如何写呀?如何取值?
class Teacher{
String id;
String name;
Set students;
}
class Student{
String id;
String name;
Set teachers;
Set books;
}
class Book{
String id;
String name;
Set students;
}

<class name="net.fengrun.lzk.stu.Book" table="books">
<id name="id" type="integer" column="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" table="student_book" inverse="true" cascade="none" lazy="true">
<key column="bookid"/>
<many-to-many column="studentid" class="net.fengrun.lzk.stu.Student"/>
</set>


</class>
<class name="net.fengrun.lzk.stu.Student" table="students">
<id name="id" type="integer" column="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="books" table="student_book" inverse="false" cascade="none" lazy="true">
<key column="studentid"/>
<many-to-many column="bookid" class="net.fengrun.lzk.stu.Book"/>
</set>
<set name="teachers" table="teacher_student" inverse="false" cascade="none" lazy="true">
<key column="studentid"/>
<many-to-many column="teacherid" class="net.fengrun.lzk.stu.Teacher"/>
</set>
</class>
<class name="net.fengrun.lzk.stu.Teacher" table="teachers">
<id name="id" type="integer" column="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" table="teacher_student" inverse="true" cascade="delete-orphan" lazy="true" >
<key column="teacherid"/>
<many-to-many column="studentid" class="net.fengrun.lzk.stu.Student"/>
</set>
</class>

Hql语句：
SELECT t FROM Teacher t join t.students s join s.books b where b.name = 'a'
解释：t.students s中s并不是集合的意思,而是t的students对象的表别名,join t.students s这个hql,hibernate会翻译成两个表的内连接关系
错误写法：
SELECT t FROM teacher t where t.students.books.name = 'a'
其实道理是很显然的,t.students是一个Set,那么这个Set怎么可能有books这样的属性呢?同理books.name也不对,所以使用表间连接并给出别名t.students s,此时的对象s才有books属性,所以可以写s.books b,最后使用b.name限制查询b.name = 'a'.
另外一种写法：
SELECT t FROM Teacher t,Student s,Book b where s.id in elements(t.students) and b.id in elements(s.books)
这种方法没有出错!不过这种方式要用子查询! 但是MySql并不支持子查询，为了通用性，选择第一种做法。
未完，待续...