problem in querying nodes (Discriminator ?) Help needed !

[Ruchi Goel]

Hi,
  I am trying a simple save and then query for a java object via 
jcrmapping.
I ahve two questions :
1. What is the concept of "discriminator " and when is it set to 
true/false ?

2. I have set discriminator = falsein mapping file.In a successive save 
and read , I am able to save a node via the following :

Ad ad = new Ad();
    ad.setPath("/ad");
    ad.setTitle("IT");
    ad.setText("This ad is for IT jobs");
    
ad.setImageAsLink("http://http://incubator.apache.org/images/apache-incubator-logo.png";);
    Collection links = new ArrayList();
    links.add((String)"http://yahoo.com";);
    links.add((String)"http://google.com";);
    ad.setLinks(links);
    try{
       FileInputStream fis = new FileInputStream("c:/tmp/Sun_logo.gif");
       ad.setImageAsBinary(fis);
    }catch(IOException ioe){
        System.out.println(ioe.getMessage());
        System.exit(0);
    }
      pm.insert(ad);
       pm.save();
, but when I try to read it back via following :
  public Ad getAd(String title){
       Filter filter = qm.createFilter(Ad.class);
       filter.addEqualTo("title", title);
       Query query = qm.createQuery(filter);
       Ad ad = (Ad)pm.getObject(query);
       return ad;
   };

The result  says :
Exception in thread "main" 
org.apache.portals.graffito.jcr.exception.PersistenceException: 
Impossible to get the object - the query returns more than one object
       at 
org.apache.portals.graffito.jcr.persistence.impl.PersistenceManagerImpl.getObject(PersistenceManagerImpl.java:454)
       at 
com.sun.portal.cms.service.AdModelServiceImpl.getAd(AdModelServiceImpl.java:54)
       at com.sun.portal.cms.Test.testReadAd(Test.java:57)
       at com.sun.portal.cms.Test.main(Test.java:69)

while I check in the repository, I have exactly one node with this title.

If I try to read the collection of objects ,
I get the following error :
Exception in thread "main" 
org.apache.portals.graffito.jcr.exception.JcrMappingException: 
Impossible to find the classdescriptor for /ad. There is no 
discriminator and associated  JCR node type
       at 
org.apache.portals.graffito.jcr.persistence.objectconverter.impl.ObjectConverterImpl.getObject(ObjectConverterImpl.java:271)
       at 
org.apache.portals.graffito.jcr.persistence.impl.PersistenceManagerImpl.getObjects(PersistenceManagerImpl.java:492)
       at 
com.sun.portal.cms.service.AdModelServiceImpl.getAd(AdModelServiceImpl.java:55)
       at com.sun.portal.cms.Test.testReadAd(Test.java:57)
       at com.sun.portal.cms.Test.main(Test.java:69)
Java Result: 1

I am sure I am missing something here , but cannot trace it . I am able 
to do teh above operations via directly using JSR170 calls.
Any help appreciated.


Thanks,
Ruchi