many-to-many used in a map

assaf · **Posted:** Wed Dec 10, 2003 9:06 am

Hello. Using Hibernate 2.0.3. I'm having problems creating a Map which represents a many-to-many relationship.

I chose a many-to-many between students and classes as a classical example.
I want a student to have a Map of classes, indexed by classId.

My mapping document looks like this:

Code:

<hibernate-mapping>
    <class
        name="tests.Student"
        table="Student"
        dynamic-update="false"
        dynamic-insert="false">

        <id name="studentId" column="studentId" type="long">
            <generator class="hilo" />
        </id>

        ...

        <map
            name="classes"
            table="StudentClass"
            lazy="true"
            sort="unsorted"
            inverse="true"
            cascade="none">
              <key column="studentId" />
              <index column="classId" type="long" />
              <many-to-many
                  class="tests.Class"
                  column="classId"
                  outer-join="auto"
              />
        </map>
    </class>
</hibernate-mapping>

The error I get is this:
net.sf.hibernate.MappingException: Repeated column in mapping for collection: tests.Student.classes column: studentId

I can see that I'm mapping both the index and the many-to-many to classId, but isn't this precisely what I want to do?

Thanks in advance,
Assaf

snek · **Joined:** Thu Dec 11, 2003 6:21 am **Posts:** 8

Don't you want a Set or a List rather than a Map for this kind of association?

emmanuel · **Posted:** Sat Dec 13, 2003 6:18 am

assaf wrote:

I can see that I'm mapping both the index and the many-to-many to classId, but isn't this precisely what I want to do?

Can't do that. The map index can be set by be user building the Map, so it cannot be mapped to an already mappede column. It has to be a new one, and it won't be related to classId, unless the application always use classId as map index.