JDO / GAE inheritance + abstract superclass NPE - google-app-engine

Question

I would like to model a simple thing but getting in trouble when reading from datastore. I found this question in different flavours but none of the answers helped in my case (using an interface instead of abstract is no option) I´ve one abstract class Media:
#SuppressWarnings("serial")
#PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
#Inheritance(strategy = InheritanceStrategy.SUBCLASS_TABLE)
public abstract class Media implements Serializable{
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#Extension(vendorName="datanucleus", key="gae.encoded-pk",
value="true")
...
#Persistent
User owner;
}
Movie is extending it.
#SuppressWarnings("serial")
#PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
public class Movie extends Media implements Serializable{
...
}
One User has a List of Media.
#SuppressWarnings("serial")
#PersistenceCapable(identityType = IdentityType.APPLICATION,
detachable="true")
public class User implements Serializable{
#PrimaryKey
#Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
#Extension(vendorName="datanucleus", key="gae.encoded-pk",
value="true")
protected String id;
#Persistent(mappedBy = "owner")
private List<Media> ownedMediaSet = new ArrayList<Media>();
}
The reading operation code is:
#Override
public List<UserDTO> readAllUser() throws IllegalArgumentException {
ArrayList<UserDTO> result = new ArrayList<UserDTO>();
PersistenceManager pm = pmf.getPersistenceManager();
Query q = pm.newQuery("select from " + User.class.getName());
List<User> res = null;
try {
res = (List<User>) q.execute();
for (User u : res) {
UserDTO uDTO = new UserDTO(u.getId(),null, u.getName(), u.getEmail());
result.add(uDTO);
}// for
} catch
This causes NPE:
java.lang.NullPointerException
at
org.datanucleus.store.appengine.DatastoreTable.addFieldMapping(DatastoreTable.java:531)
at org.datanucleus.store.appengine.DatastoreTable.initializeNonPK(DatastoreTable.java:440)
I dont get it. Without Media being abstract everything works fine. Maybe someone knows about the problem and can give me a hint.
Regards

Answer 1

You can not make a list of Media... because there is no instantiable class of media.
--> that means there is no "database table" media
Polymorph relationship doesen't work with GAE...
https://developers.google.com/appengine/docs/java/datastore/jdo/relationships#Polymorphic_Relationships
#Persistent(mappedBy = "owner")
private List<Media> ownedMediaSet = new ArrayList<Media>();
Make the Class Media not abstract then it works.
Or you make a List of movies...
#Persistent(mappedBy = "owner")
private List<Movie> ownedMediaSet = new ArrayList<Movie>();
but thats probably not what you want.
so the last option is what's in this artikle:
https://developers.google.com/appengine/docs/java/datastore/jdo/relationships#Polymorphic_Relationships
make a list of Keys:
#Persistent
private List<Key> ownedMediaSet = new ArrayList<Key>();

Answer 2

Try v2.0 of Googles JDO plugin and see how that goes. Likely it does nothing different yet, but if that is the case you can easily raise an issue with simple testcase at http://code.google.com/p/datanucleus-appengine/issues/list The fact is the v1 plugin did some things in illogical ways (see Sam's answer for some links that this illogical handling caused). You could also just set inheritance strategy to COMPLETE_TABLE since that is all that is really supported with BigTable (i.e each class has a "Kind" that holds all properties for that type).