Google Datastore - Ensure only one entity is created for an entity with a specific ancestor key

Question

I want to ensure that only one entity is created for an entity with a specific ancestor Key. My solution is to put the ancestor query inside a transaction, check if the entity exists, and if not, create it.

Will this ensure only one entity exists with a specific ancestor key?

ofy().transact(new VoidWork() {
    public void vrun() {

            Entity entity = ofy().load().type(Entity.class)
                    .ancestor(ancestorKey)
                    .first()
                    .now();

            if (entity == null) {
                // Entity does not exist. Create it.
                final Entity newEntity = new Entity(ancestorKey);

                ofy().save().entity(newEntity).now();
            } else {
                // Entity already exists.
            }

        }
    }
});

Answer 1

SO doesn't like one-word answers so I can't just say "Yes".

If you're just trying to ensure that one and only one thing exists, you don't need the ancestor query - just do a get-by-key on a known id.

And since Java7 is gone, no reason not to put this in a lambda.