Ensuring a given key does not exists prior to "put"

[Mike Gleason jr Couturier]

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

[Mike Gleason jr Couturier]

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

[Mike Gleason jr Couturier]

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, "jo...@doe.com");

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

[Robert Kluin]

If you want it to be unique you'll need to use the keyname, like your earlier idea. Fetch by key, if it doesn't exist you're ok to create it.

Robert

> --
> You received this message because you are subscribed to the Google Groups "Google App Engine" group.
> To post to this group, send email to google-a...@googlegroups.com.
> To unsubscribe from this group, send email to google-appengi...@googlegroups.com.
> For more options, visit this group at http://groups.google.com/group/google-appengine?hl=en.
>

[Mike Gleason jr Couturier]

I'll do as you suggest, thanks!

On Aug 24, 2:12 am, Robert Kluin <robert.kl...@gmail.com> wrote:
> If you want it to be unique you'll need to use the keyname, like your earlier idea. Fetch by key, if it doesn't exist you're ok to create it.
>
> Robert
>

> On Aug 24, 2011, at 0:12, Mike Gleason jr Couturier <mikegleaso...@gmail.com> wrote:
>
>
>
>
>
>
>
> > 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, "j...@doe.com");

[John OBrien]

there is also  get_or_insert http://code.google.com/appengine/docs/python/datastore/modelclass.html#Model_get_or_insert