google app engine - Backward pagination with cursor is working but missing an item

Question

From looking for ideas/alternatives to providing a page/item count/navigation of items matching a GAE datastore query, I could find a hint how to backward page navigation with a single cursor by REVERSING ORDER.

class CursorTests(test_utils.NDBTest):

  def testFirst(self):
    class Bar(model.Model):
      value = model.IntegerProperty()

    self.entities = []
    for i in range(10):
        e = Bar(value=i)
        e.put()
        self.entities.append(e)

    q = Bar.query()
    bars, next_cursor, more = q.order(Bar.key).fetch_page(3)
    barz, another_cursor, more2 = q.order(-Bar.key).fetch_page(3, start_cursor=next_cursor)
    self.assertEqual(len(bars), len(barz))

Unfortunately it failed with this error.

Traceback (most recent call last): File "/Users/reiot/Documents/Works/appengine-ndb-experiment/ndb/query_test.py", line 32, in testFirst self.assertEqual(len(bars), len(baz)) AssertionError: 3 != 2

Yes, an item in boundary is missing with reverse query.

bars = [Bar(key=Key('Bar', 1), value=0), Bar(key=Key('Bar', 2), value=1), Bar(key=Key('Bar', 3), value=2)] 
bars = [Bar(key=Key('Bar', 2), value=1), Bar(key=Key('Bar', 1), value=0)]

How can I fix this problem?

Answer 1

Waitting for answers