Create-or-Err with Objectify

Question

I\'m getting started with Google App Engine, and I\'m using Objectify. How do I create a root entity in the data store, but err if it already exists? I didn\'t find anythi

Answer 1

This should guarantee consistent behavior:

final String id = // pick the unique id
final long txnId = // pick a uuid, timestamp, or even just a random number

ofy().transact(new VoidWork() {
    public void vrun() {
        Thing th = ofy().load().type(thing.class).id(id).now();
        if (th != null) {
            if (th.getTxnId() == txnId)
                return;
            else
                throw ThingAlreadyExistsException();
        }

        th = createThing(id, txnId);
        ofy().save().entity(th);
    }
});