And let's say I do it on a property, is this would be enough?
Query q = new Query("User').setKeysOnly();
q.addFilter("Email", Query.FilterOperator.EQUAL,
"[email protected]");
Transaction tx = datastore.beginTransaction();
if
(datastore.prepare(q).countEntities(FetchOptions.Builder.withDefaults())
> 0)
{
tx.rollback();
return false;
}
datastore.put(entity);
tx.commit();
return true;
On Aug 23, 11:23 pm, Mike Gleason jr Couturier
<[email protected]> wrote:
> I've done this, I just wan to have a second opinion:
>
> Entity entity = new Entity("User", "mike"
> /*username*/);
> Transaction tx = datastore.beginTransaction();
>
> try {
> datastore.get(entity.getKey());
> return false; // error, already in hrd
> } catch (EntityNotFoundException e) {
> datastore.put(entity);
> tx.commit();
> return true;
> } finally {
> if (tx.isActive()) {
> tx.rollback();
> }
> }
>
> Seems a bit overkill but we need consistency right? :)
>
> Thanks
>
> On Aug 23, 10:39 pm, Mike Gleason jr Couturier
>
>
>
>
>
>
>
> <[email protected]> wrote:
> > Hi,
>
> > Given the nature of the "put" function (low level hrd API), how can I
> > atomically put a new entity without overwriting an existing one if the
> > keys are chosen by the users (String values)?
>
> > Let's say I have 2 simultaneous users that creates at the same time an
> > entity of kind "User" with the custom key "johndoe" (because the user
> > name is the key), I don't want the second one to overwrite the
> > first...
>
> > I don't want to have a property for the username since I'll have the
> > same "unique index" problem.
>
> > Is a transaction with a:
> > 1) get
> > 2) catch EntityNotFoundException
> > 3) put
> > 4) commit
>
> > enough to ensure consistency ?
>
> > Thanks
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