[ojb-users] Problems with M-N mappings (newbie)

[Martin F. <fu...@be...>]

Hi all,

I've just started using objectbridge (the 0.9 version) and am having a few 
problems with M-N mappings :

1) Unable to perform non-decomposed N:M mapping
======================================

I have two classes StringFDP and AndFDP.
StringFDP just contains a string field, wheras AndFDP contains a Collection 
(of StringFDPs) (field name m_fdps)
I also added a Collection field (named _containers) to StringFDP (although 
I don't really want this - see below)

Following the test cases I did this :

    <class-descriptor
    	  class="prestudy.persist.bo.StringFdp"
    	  table="STRING_FDP"
    >
       <field-descriptor id="1"
          name="_uid"
          column="objid"
          jdbc-type="INTEGER"
          primarykey="true"
          autoincrement="true"
       />
       <field-descriptor id="2"
          name="m_val"
          column="val"
          jdbc-type="VARCHAR"
       />
       <collection-descriptor
          name="_containers"
          element-class-ref="prestudy.persist.bo.AndFdp"
          indirection-table="AND_FDP_ASSOC"
       >
          <fk-pointing-to-this-class column="FDP_ID"/>
          <fk-pointing-to-element-class column="AND_ID"/>
       </collection-descriptor>
    </class-descriptor>


    <class-descriptor
    	  class="prestudy.persist.bo.AndFdp"
    	  table="AND_FDP"
    >
       <field-descriptor id="1"
          name="_uid"
          column="objid"
          jdbc-type="INTEGER"
          primarykey="true"
          autoincrement="true"
       />
       <collection-descriptor
          name="m_fdps"
          element-class-ref="prestudy.persist.bo.StringFdp"
          indirection-table="AND_FDP_ASSOC"
       >
          <fk-pointing-to-this-class column="AND_ID"/>
          <fk-pointing-to-element-class column="FDP_ID"/>
       </collection-descriptor>

    </class-descriptor>

This gave me a NPE during a lock operation (I had to modify TransactionImpl 
to print the cause of the exception rather than just the 
LockNotGrantedException) :

java.lang.NullPointerException
         at 
ojb.broker.metadata.ObjectReferenceDescriptor.getForeignKeyFieldDescr
iptors(ObjectReferenceDescriptor.java:250)
         at ojb.odmg.TransactionImpl.lockCollections(Unknown Source)
         at ojb.odmg.TransactionImpl.register(Unknown Source)
         at ojb.odmg.TransactionImpl.lock(Unknown Source)
         at prestudy.persist.ojb.OjbFdpStorer.save(OjbFdpStorer.java:72)
         at prestudy.persist.ojb.OjbFdpStorer.test(OjbFdpStorer.java:119)
         at prestudy.persist.ojb.OjbFdpStorer.main(OjbFdpStorer.java:94)
org.odmg.LockNotGrantedException
         at ojb.odmg.TransactionImpl.lock(Unknown Source)
         at prestudy.persist.ojb.OjbFdpStorer.save(OjbFdpStorer.java:72)
         at prestudy.persist.ojb.OjbFdpStorer.test(OjbFdpStorer.java:119)
         at prestudy.persist.ojb.OjbFdpStorer.main(OjbFdpStorer.java:94)

It seems OJB wants a foreign key definition. But the unit test case the 
non-decomposed mapping PEOPLE, PROJECTS using indirection_table works fine. 
The only difference I can see is that in the unit test there is _also_ a 
decomposed mapping between the same two classes (via ROLE).
In tutorial 3 a seemingly dummy reverse_fk_descriptor is used (999) but 
when I add this I get an ArrayIndexOutOfBoundsException.


2) How do I do a unidirectional N:M mapping?
=================================
In fact my object model only needs navigability from AndFdp to StringFdp 
and not the inverse. Why do I have to add an unneeded Collection to my 
StringFdp class.

3) What I'm really trying to do.
======================
In fact I'm really trying to map a composite pattern structure of objects 
where each object is either a simple object (such as StringFdp) or a 
composite (such as AndFdp). The idea being to have several composites 
representing logical operations (Or, Nand etc).

Is it possible to map this type of model?


Help much appreciated,

Martin Fuzzey