n:m mapping problem

Hi all,

I have a problem when using m:n mapping. I will describe it below.

I just implement a tutorial in this link
http://db.apache.org/ojb/docu/guides/basic-technique.html#Support+for+Non-Decomposed+m%3An+Mappings

There are three table: PERSON, PROJECT, PROJECT_PERSON. PROJECT_PERSON is
the connector standing for m:n relation between PROJECT and PERSON.

PROJECT(id,name)
PERSON(id,name)
PROJECT_PERSON(project_id, person_id)

The mapping file as below:____________________________

<!--           vn.com.tma.testOJB.Person              -->
<class-descriptor
    class="vn.com.tma.testOJB.Person"


    table="person"
>
    <field-descriptor
        name="id"
        column="id"
        jdbc-type="INTEGER"
        primarykey="true"
        autoincrement="true"
    />
    <field-descriptor
        name="name"
        column="name"
        jdbc-type="VARCHAR"
    />


    <collection-descriptor
        name="projects"
        collection-class="
org.apache.ojb.broker.util.collections.ManageableArrayList"
        element-class-ref="vn.com.tma.testOJB.Project"
        auto-retrieve="true"
        auto-update="true"
        indirection-table="PROJECT_PERSON"
    >
        <fk-pointing-to-this-class column="PERSON_ID"/>
        <fk-pointing-to-element-class column="PROJECT_ID"/>
    </collection-descriptor>

</class-descriptor>

<!--           vn.com.tma.testOJB.Project              -->
<class-descriptor
    class="vn.com.tma.testOJB.Project"

    table="project"
>
    <field-descriptor
        name="id"
        column="id"
        jdbc-type="INTEGER"
        primarykey="true"
        autoincrement="true"
    />
    <field-descriptor
        name="name"
        column="Name"
        jdbc-type="VARCHAR"
    />
    <collection-descriptor
        name="employees"
        collection-class="
org.apache.ojb.broker.util.collections.ManageableArrayList"
        element-class-ref="vn.com.tma.testOJB.Person"
        auto-retrieve="true"
        auto-update="false"
        indirection-table="PROJECT_PERSON"
    >
        <fk-pointing-to-this-class column="PROJECT_ID"/>
        <fk-pointing-to-element-class column="PERSON_ID"/>
    </collection-descriptor>
</class-descriptor>

When I run these codes, it has error:
        PersistenceBroker broker =
PersistenceBrokerFactory.defaultPersistenceBroker();
        Identity oid = broker.serviceIdentity().buildIdentity(Person.class,
new Integer(1));
        Person person = (Person) broker.getObjectByIdentity(oid);

The error is caused by the third line. This is the exception stack:

Exception in thread "main" org.apache.ojb.broker.PersistenceBrokerException:
java.lang.NullPointerException
    at org.apache.ojb.broker.core.QueryReferenceBroker.getCollectionByQuery(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.doRetrieveCollection(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.retrieveCollection(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.doRetrieveCollections(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.retrieveCollections(Unknown
Source)
    at org.apache.ojb.broker.core.PersistenceBrokerImpl.getDBObject(Unknown
Source)
    at
org.apache.ojb.broker.core.PersistenceBrokerImpl.doGetObjectByIdentity(Unknown
Source)
    at org.apache.ojb.broker.core.PersistenceBrokerImpl.getObjectByIdentity(Unknown
Source)
    at
org.apache.ojb.broker.core.DelegatingPersistenceBroker.getObjectByIdentity(Unknown
Source)
    at
org.apache.ojb.broker.core.DelegatingPersistenceBroker.getObjectByIdentity(Unknown
Source)
    at vn.com.tma.main.Run.main(Run.java:41)
Caused by: java.lang.NullPointerException
    at
org.apache.ojb.broker.accesslayer.MtoNCollectionPrefetcher.associateBatched(Unknown
Source)
    at
org.apache.ojb.broker.accesslayer.MtoNCollectionPrefetcher.prefetchRelationship(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.performRetrievalTasks(Unknown
Source)
    at org.apache.ojb.broker.core.QueryReferenceBroker.getCollectionByQuery(Unknown
Source)
    ... 11 more

But if I change auto-archive to "false" in arbitrary one of two
class-descriptor, there is no error anymore. But it still causes the same
error when I use method "broker.retrieveAllReferences()" on an instance of
the rest class-descriptor.

Only when I change auto-archive of both class-descriptor to "false", the
error is omitted.

Can anyone explain why and help me resolve the problem when using
auto-archive="true" on both class-descriptor?
Thank you very for your help.

-- 
Regards,
Truong Nguyen Huy