Question

How can I retreive only one entity from a query on Google App Engine. I’m creating a query for with ORDER BY and using:

List<Object> objs= (List<Object>) pm.newQuery(query).execute();
if (objs.size() > 0) {
    obj= objs.get(0);
}

But, when I have a large amount of “Objs” in Database, the query is too expensive, and I’m having too much delay.

There is another way to do this query?

Answer 1

Have a look at this document.

These are valid usages of a query:

// Construct then prepare your query
List<Entity> get5TallestPeople() {
    DatastoreService ds = DatastoreServiceFactory.getDatastoreService();
    Query q = new Query("Person");
    q.addSort("height", SortDirection.DESCENDING);
    PreparedQuery pq = ds.prepare(q);
    return pq.asList(FetchOptions.Builder.withLimit(5));
}

and

return pq.asList(FetchOptions.Builder.withLimit(5).offset(5)));

In your case I guess you can use the latter syntax.

It uses low level Java API rather than JDO, but it shouldn’t put you off from using it.