Paging through results on google app engine

The sample code given on the appengine article about paging through results is incomplete for the method of paging through a results set by item key. Here is an example that will page forward and backward through a result set by item key. This example will display items oldest first. The next link will bring you to the set of next newest items.

template code for displaying the results, let's say it's called listitems.html

the relevant code in your application file.

 
#assume you have imported all of the googley goodness

#this sets the paging size
FETCHLIMIT = 10

# an empty class for testing against.
class Item(db.Model):
    created = db.DateTimeProperty(auto_now_add=True)

class ListItems(webapp.RequestHandler):
    def get(self):
        query = Item.all()
        items = query.fetch(FETCHLIMIT+1)
        
        forward = self.request.get("next")
        back = self.request.get("previous")

        if forward:
            items = Item.all().order("__key__").filter('__key__ >=', db.Key(forward)).fetch(FETCHLIMIT+1)
        elif back:
            items = Item.all().order("-__key__").filter('__key__ <=', db.Key(back)).fetch(FETCHLIMIT+1)
            items.reverse()
            # in order to go backwards through the set we need to revers the order so that we get the newest keys on top
            # the filter than drops the items we have already seen
            # but to get the previous link to pass the oldest key in the set we need to reverse the items before heading on
        else:
            items = Item.all().order("__key__").fetch(FETCHLIMIT+1)     
       # the key part here missing from the example code is making the right call against the right passed variable
       # forward = self.request.get("next") returns a string, we convert that to a key object with the line:
       # db.Key(forward)
             
        # do we show next and previous links?
        # start with none
        next = None
        previous = None 
        # we need to get the key of the very first item in the results set, 
        query = Item.all()
        first_item_key = query.fetch(1)[0].key()
        previous = activities[0].key()
        # if previous key is the same as the key of the first item in the class then don't display a previous link
        if previous == first_item_key: previous = None 

        # we have just asked for a batch of items. if the returned set is bigger than the amount we want on a page
        # we know there are some results left to get so we set a next link
        if len(items) == FETCHLIMIT+1:
            next = items[-1].key()
            items = items[:FETCHLIMIT]
        
        # perhaps we want to display the total count of objects in the datastore
        item_number = Item.all().count()
        template_values = {'count':item_number, 'items':items, 'next':next, 'previous':previous}
        path = os.path.join(os.path.dirname(__file__), 'listitems.html')
        self.response.out.write(template.render(path, template_values))

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