http://groups.google.com/group/google-appengine-java/browse_thread/thread/fd8043e63ff6c22e/
the link above works only for persisting and retrieving hash maps ,but i did
not find any "proper" solution for updating and persisting the updated
hashmaps.
"You have to provide a new reference to the updated hashmap ,since datastore
cannot identify between updated and stale hashmap."
But it did not work in my case and i urge you to try the same before you try
alternatives.
The Alternative i found was ,
1) retrieve the persisted hashmap ,
2) create a new hashmap reference to updated hashmap
3) make the new reference persistent.
4)delete the previous reference.
code: /* i have used some custom classes ,but the you should be able to
understand the logic i've employed to get the solution */
/*Retrieve the persisted HashMap*/
PersistenceManager pm0= PMF.get().getPersistenceManager();
Query query0 = pm0.newQuery(VocabHashMap.class);
List<VocabHashMap> results1 = (List<VocabHashMap>) query0.execute();
/*if there are no hashmaps, create a new one */
if (results1.isEmpty())
{
HashMap<String, Double> map = new HashMap<String, Double>();
for (String a : linelist) {
Double freq = map.get(a);
map.put(a, (freq == null) ? 1 : freq + 1);
}
VocabHashMap vhm =new VocabHashMap(map);
pm0.makePersistent(vhm);
pm0.close();
}
else{
for (VocabHashMap vhm : results1) {
/*if you already have a hashmap,then update it */
HashMap<String,Double> map1=vhm.getMap(); /* assign a new reference to
retrieved hashmap */
for (String a : linelist) {
Double freq = map1.get(a);
map1.put(a, (freq == null) ? 1 : freq + 1);
}
VocabHashMap vhm1=new VocabHashMap(map1); /* create a new hashmap*/
pm0.makePersistent(vhm1); /*save the updated one */
pm0.deletePersistent(vhm); /*delete the old reference */
}
pm0.close();
}
On Sun, Apr 18, 2010 at 10:41 AM, seleronm <seler...@gmail.com> wrote:
> Hi,
>
> I was useful referring to this thread.
> You might be also useful for it.
>
>
> http://groups.google.com/group/google-appengine-java/browse_thread/thread/fd8043e63ff6c22e/
>
> Hope some of this helps.
>
> thanks.
>
> > I am not able to persist a HashMap field. This is how I define it:
> >
> > @Persistent(serialized = "true", defaultFetchGroup = "true")
> > private Map<String, Integer> items;
> >
> > When I create the Object which the Map is a field of, I instantiate
> > the Map as follows:
> >
> > items = new HashMap<String, Integer>();
> >
> > When I update the "items" HashMap I try to persist the object by doing
> > the following:
> >
> > PersistenceManager pm = PMF.get().getPersistenceManager();
> > // Query for Object (HashMap is a field of this Object)
> > // Update HashMap
> > pm.makePersistent(objectContainingHashMap);
> >
> > However, when I attempt to retrieve the object after persisting it and
> > read the HashMap it is always empty, as though I never updated it.
> >
> > Has anyone experienced this before?
> >
> > Thanks
> >
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