Attempt to fix len iteration bug

mongoengine/queryset/queryset.py

@@ -75,7 +75,8 @@ def _iter_results(self):
                 yield self._result_cache[pos]
                 pos += 1
             if not self._has_more:
-                raise StopIteration
+                if pos == len(self):
+                    raise StopIteration
             if len(self._result_cache) <= pos:
                 self._populate_cache()
 
@@ -155,6 +156,8 @@ def __iter__(self):
         queryset.rewind()
         return queryset
 
+    def __len__(self):
+        return len([1 for i in self])
 
 class QuerySetNoDeRef(QuerySet):
     """Special no_dereference QuerySet"""

tests/queryset/queryset.py

@@ -4822,5 +4822,30 @@ class Doc(Document):
 
         self.assertEqual(1, Doc.objects(item__type__="axe").count())
 
+    def test_len_during_iteration(self):
+        """
+        Tests that calling len on a queyset during iteration doesn't stop
+        paging.
+        """
+
+        class Data(Document):
+            pass
+
+        for i in xrange(500):
+            Data().save()
+
+        records = Data.objects.limit(250)
+        len(records)
+        for i, r in enumerate(records):
+            if i == 58:
+                len(records)
+        self.assertEqual(i, 249)
+
+        records = Data.objects.limit(250)
+        for i, r in enumerate(records):
+            if i == 58:
+                len(records)
+        self.assertEqual(i, 249)
+
 if __name__ == '__main__':
     unittest.main()