Objectify paging with Cursors

Question

I have this method in my RPC service:

@Override
    public Entrata[] getEntrate(int from, int to) {
        List<Entrata> data = entrateDao.list();
        return data.toArray(new Entrata[0]);
    }

As you can see, I am not using the two parameters, which, in a SQL world, I would use as LIMIT and OFFSET.

It's not completely clear what I have to do now, I started reading this: http://code.google.com/p/objectify-appengine/wiki/IntroductionToObjectify#Cursors

I think I have to do a query.startCursor()

Then iterate for "TO" times, the page size.

All right? Can you help me with some snippets? :)

Answer 1

From docs: Cursors let you take a "checkpoint" in a query result set, store the checkpoint elsewhere, and then resume from where you left off late

As you need just limit/offset, you have to use limit() and offset() method of Objectify Query. Like:

ob.query(Entrata.class).limit(to - from).offset(from)

Or, when you have cursor:

String cursor = // get it from request
Query<Entrata> query = ob.query(Entrata.class);
Query q = query.startCursor(Cursor.fromWebSafeString(cursor));
q.limit(x);
QueryResultIterator<Entrate> iterator = query.iterator()
List<Entrate> data = // fetch data
String newCursor = iterrator.getStartCursor().toWebSafeString()
return new EntrataListWithCursor(data, cursor);

Answer 2

I just want make sure you don't have any errors in your code since you can copy and past the Igor Artamonov code. Here is a cleaner code from Objectify Wiki with less errors and some documentation:

// create the query and set the limit to 1000
Query<Car> query = ofy().load().type(Car.class).limit(1000);

// Here you get the cursor (if exists) from the request
// For the first request, i-e the first page, this parameter(cursor) will be null
String cursorStr = request.getParameter("cursor");

// Here you check if cursor is not null and not empty
// If so, we start our query from the last check point
if (cursorStr != null && !cursorStr.isEmpty())
    query = query.startAt(Cursor.fromWebSafeString(cursorStr));

// We need this variable to know when we have been loaded all the entries
boolean remaining = false;
QueryResultIterator<Car> iterator = query.iterator();
while (iterator.hasNext()) {
    Car car = iterator.next();

    ... // your code here

    // We have found entries, so we set this variable to true.
    // That means, we have probably another page to fetch
    remaining = true;
}

// If we have found entries, we send the last check point
if (remaining) {
    // we take the last check point by calling "toWebSafeString()" from the iterator's cursor
    Cursor cursor = iterator.getCursor();
    Queue queue = QueueFactory.getDefaultQueue();
    queue.add(url("/pathToThisServlet").param("cursor", cursor.toWebSafeString()));
}