This depends on what you mean by "lazily load" the entity. There are at least three possible interpretations of lazy loading:
1) Lazily fetching data across the wire 2) Lazily converting from protocol buffers to Entity objects 3) Lazily translating Entity objects to Java POJOs The first two steps are encapsulated within the Low-Level API, the third step is part of JDO/Objectify/etc. I know that when you issue a query, the results are batched in chunks of a size that you can control. #1 and #2 are done together; the data is pulled across the wire into materialized Entity objects in increments of the batch size. Objectify does #3 at the last possible moment; if you do not call next() and retrieve a POJO entity, the POJO will not be instantiated. You can call hasNext() to determine the existence of another page and this will not create a POJO - however, the Low-Level API will create the Entity. In practice, it's hard to imagine that this matters unless your page size is something like 2. If you have 20 elements on a page, the worst-case scenario is that fetching 21 will add 5% to the cost of the query. That's not much. You can do what you want with Objectify and I suspect you can do what you want with JDO, just don't use getResultList(). Get an actual QueryResultIterator so the getCursor() call will return the correct location. Jeff On Thu, May 6, 2010 at 8:48 PM, Chau Huynh <cmhu...@gmail.com> wrote: > Hi Jeff, > > With JPA I will need to excute getResultList() to get a collection > http://java.sun.com/javaee/5/docs/api/javax/persistence/Query.html#getResultList() > Somehow org.datanucleus.store.appengine.query.getCursor(Iterator<?> iter) > can not get the cursor in the middle of my iterator, a null value is > returned in sample code below. > I guess getCursor() has something to do with the List returned. > > EntityManager em = > Query query = em.createQuery(queryString); > query.setMaxResults(pageSize + 1); > em.getTransaction().begin(); > Iterator<TestEntity> iterator = query.getResultList().iterator(); > List<TestEntity> list = new ArrayList<TestEntity>(); > for (int i = 0; (i < pageSize) && iterator.hasNext(); i++) { > list.add(iterator.next()); > } > Cursor cursor = JPACursorHelper.getCursor(iterator); > // assert iterator.hasNext() == true and cursor == null > em.getTransaction().rollback(); > > Just my curious: > if I'm able to iterate through the result (using > com.google.appengine.api.datastore.QueryResultIterator<T>) > is it true that each iterator.hasNext() will lazily load entity from the > data store? > My concern is it might cost me more time that way than using > Query.getResultList(). Please advise. > > Thank you. > > On Fri, May 7, 2010 at 2:24 AM, Jeff Schnitzer <j...@infohazard.org> wrote: >> >> I'm a little confused by this. Why doesn't the OP's approach work? >> >> 1) Create query, fetch with limit of 21 items >> 2) Iterate through 20 items >> 3) Get cursor to be used for next page (if necessary) >> 4) Iterate to 21st item just to check for existence >> >> Why won't this work? The documentation seems to imply that this is >> how cursors work... >> >> Jeff >> >> On Thu, May 6, 2010 at 1:03 AM, Ikai L (Google) <ika...@google.com> wrote: >> > This is a datastore limitation and not related to JDO/JPA, so you won't >> > be >> > able to do this in Objectify either (you can look at the low-level API, >> > it >> > doesn't provide this functionality). I believe I saw an issue in the >> > issues >> > tracker about creating cursors from arbitrary locations. >> > >> > -- >> > You received this message because you are subscribed to the Google >> > Groups >> > "Google App Engine for Java" group. >> > To post to this group, send email to >> > google-appengine-j...@googlegroups.com. >> > To unsubscribe from this group, send email to >> > google-appengine-java+unsubscr...@googlegroups.com. >> > For more options, visit this group at >> > http://groups.google.com/group/google-appengine-java?hl=en. >> > >> >> -- >> You received this message because you are subscribed to the Google Groups >> "Google App Engine for Java" group. >> To post to this group, send email to >> google-appengine-j...@googlegroups.com. >> To unsubscribe from this group, send email to >> google-appengine-java+unsubscr...@googlegroups.com. >> For more options, visit this group at >> http://groups.google.com/group/google-appengine-java?hl=en. >> > > -- > You received this message because you are subscribed to the Google Groups > "Google App Engine for Java" group. > To post to this group, send email to google-appengine-j...@googlegroups.com. > To unsubscribe from this group, send email to > google-appengine-java+unsubscr...@googlegroups.com. > For more options, visit this group at > http://groups.google.com/group/google-appengine-java?hl=en. > -- You received this message because you are subscribed to the Google Groups "Google App Engine for Java" group. 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