Hey, Relationships in GAE are a bit picky. It is my understanding that you can only persist children by way of persisting their parents. So, in your case, both Race and Runner are children of Result and you would only persist the two result objects - the dependent children would be persisted automatically. I'm willing to bet, though, that a single race object cannot be the child of two different parent objects, though I'm not certain. The details are all here: http://code.google.com/appengine/docs/java/datastore/relationships.html
For the most part, I've found that parent/child relationships are only useful when the two are highly related - usually when I'm extending a class to provide more data. (e.g. Race -> RaceDetails). The "easy" way to get around this is to store the objects in unowned relationships, referencing other objects by IDs as described in http://code.google.com/appengine/docs/java/datastore/relationships.html#Unowned_Relationships. Then, with some fancy getters/setters that automatically query the Datastore, you end up with the same result. See my example below. If anyone has any other advice, though, please contribute. I haven't found a great way to handle this and I'm trying to avoid using GAE specific libraries to help. Jake public class Race { private Key id; ... } public class Result { private Key id; private Key raceId; public Race getRace() { //query datastore with getObjectById(Race.class, raceId); } } On Feb 6, 7:15 pm, Rodolphe <[email protected]> wrote: > Hello, > > I am very new to appengine, and I am trying a very basic model exemple: A > Race class is linked to a Runner class through a Result class > > When I am running the folloging code (the full code is attached): > Race race = new Race("Paris", new Date(), 10); > > Runner runner1 = new Runner("Smith"); > Runner runner2 = new Runner("John"); > > Result result1 = new Result(race, runner1, 1); > Result result2 = new Result(race, runner2, 2); > > PersistenceManager pm = PMF.get().getPersistenceManager(); > try { > race = pm.makePersistent(race); > > runner1 = pm.makePersistent(runner1); > runner2 = pm.makePersistent(runner2); > > pm.makePersistent(result1); > pm.makePersistent(result2); > } finally { > pm.close(); > } > > I get the following error: > Detected attempt to establish Result(4) as the parent of Runner(2) but the > entity identified by Runner(2) has already been persisted without a parent. > A parent cannot be established or changed once an object has been persisted. > org.datanucleus.store.appengine.FatalNucleusUserException: Detected attempt > to establish Result(4) as the parent of Runner(2) but the entity identified > by Runner(2) has already been persisted without a parent. A parent cannot > be established or changed once an object has been persisted. > at > org.datanucleus.store.appengine.DatastoreRelationFieldManager.checkForParentSwitch(DatastoreRelationFieldManager.java:204) > at > org.datanucleus.store.appengine.DatastoreRelationFieldManager$1.setObjectViaMapping(DatastoreRelationFieldManager.java:125) > at > org.datanucleus.store.appengine.DatastoreRelationFieldManager$1.apply(DatastoreRelationFieldManager.java:104) > at > org.datanucleus.store.appengine.DatastoreRelationFieldManager.storeRelations(DatastoreRelationFieldManager.java:78) > at > org.datanucleus.store.appengine.DatastoreFieldManager.storeRelations(DatastoreFieldManager.java:812) > at > org.datanucleus.store.appengine.DatastorePersistenceHandler.insertPostProcess(DatastorePersistenceHandler.java:288) > at > org.datanucleus.store.appengine.DatastorePersistenceHandler.insertObjects(DatastorePersistenceHandler.java:241) > at > org.datanucleus.store.appengine.DatastorePersistenceHandler.insertObject(DatastorePersistenceHandler.java:225) > at > org.datanucleus.state.JDOStateManagerImpl.internalMakePersistent(JDOStateManagerImpl.java:3185) > at > org.datanucleus.state.JDOStateManagerImpl.makePersistent(JDOStateManagerImpl.java:3161) > at > org.datanucleus.ObjectManagerImpl.persistObjectInternal(ObjectManagerImpl.java:1298) > at > org.datanucleus.ObjectManagerImpl.persistObject(ObjectManagerImpl.java:1175) > at > org.datanucleus.jdo.JDOPersistenceManager.jdoMakePersistent(JDOPersistenceManager.java:669) > at > org.datanucleus.jdo.JDOPersistenceManager.makePersistent(JDOPersistenceManager.java:694) > at race.TestRace.test2(TestRace.java:58) > > Any idea how I could solve this ? > > Thank you in advance > Rodolphe > > TestRace.java > 3KViewDownload > > Race.java > 1KViewDownload > > Result.java > 1KViewDownload > > Runner.java > 1KViewDownload -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/google-appengine-java?hl=en.
