improve doc + changelog

docs/changelog.rst

@@ -6,7 +6,8 @@ Changelog
 Development
 ===========
 - (Fill this out as you fix issues and develop your features).
-- Fixed a bug that made the queryset drop the read_preference after clone(). 
+- Fix a bug that made the queryset drop the read_preference after clone().
+- Fix the behavior of Doc.objects.limit(0) which should return all documents (similar to mongodb) #2311
 
 Changes in 0.20.0
 =================

mongoengine/queryset/base.py

@@ -83,7 +83,7 @@ class BaseQuerySet:
         self._cursor_obj = None
         self._limit = None
         self._skip = None
-        self._empty = False
+
         self._hint = -1  # Using -1 as None is a valid value for hint
         self._collation = None
         self._batch_size = None
@@ -91,6 +91,13 @@ class BaseQuerySet:
         self._max_time_ms = None
         self._comment = None
 
+        # Hack - As people expect cursor[5:5] to return
+        # an empty result set. It's hard to do that right, though, because the
+        # server uses limit(0) to mean 'no limit'. So we set _empty
+        # in that case and check for it when iterating. We also unset
+        # it anytime we change _limit. Inspired by how it is done in pymongo.Cursor
+        self._empty = False
+
     def __call__(self, q_obj=None, **query):
         """Filter the selected documents by calling the
         :class:`~mongoengine.queryset.QuerySet` with a query.

tests/queryset/test_queryset.py

@@ -409,6 +409,9 @@ class TestQueryset(unittest.TestCase):
 
         assert list(A.objects.none()) == []
         assert list(A.objects.none().all()) == []
+        assert list(A.objects.none().limit(1)) == []
+        assert list(A.objects.none().skip(1)) == []
+        assert list(A.objects.none()[:5]) == []
 
     def test_chaining(self):
         class A(Document):