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?

classTeacher{

Stringid;

Stringname;

Setstudents;

}

classStudent{

Stringid;

Stringname;

Setteachers;

Setbooks;

}

classBook{

Stringid;

Stringname;

Setstudents;

}

<classname="net.fengrun.lzk.stu.Book"table="books">

<idname="id"type="integer"column="id">

<generatorclass="native"/>

</id>

<propertyname="name"/>

<setname="students"table="student_book"inverse="true"cascade="none"lazy="true">

<keycolumn="bookid"/>

<many-to-manycolumn="studentid"class="net.fengrun.lzk.stu.Student"/>

</set>

</class>

<classname="net.fengrun.lzk.stu.Student"table="students">

<idname="id"type="integer"column="id">

<generatorclass="native"/>

</id>

<propertyname="name"/>

<setname="books"table="student_book"inverse="false"cascade="none"lazy="true">

<keycolumn="studentid"/>

<many-to-manycolumn="bookid"class="net.fengrun.lzk.stu.Book"/>

</set>

<setname="teachers"table="teacher_student"inverse="false"cascade="none"lazy="true">

<keycolumn="studentid"/>

<many-to-manycolumn="teacherid"class="net.fengrun.lzk.stu.Teacher"/>

</set>

</class>

<classname="net.fengrun.lzk.stu.Teacher"table="teachers">

<idname="id"type="integer"column="id">

<generatorclass="native"/>

</id>

<propertyname="name"/>

<setname="students"table="teacher_student"inverse="true"cascade="delete-orphan"lazy="true">

<keycolumn="teacherid"/>

<many-to-manycolumn="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...

转自:http://ysongren.blog.163.com/blog/static/615422072008111082032493/