Fix queryset batch_size that wasn't copied to cloned queryset

docs/changelog.rst

@@ -6,6 +6,7 @@ Development
 ===========
 - (Fill this out as you fix issues and develop your features).
 - Fix .only() working improperly after using .count() of the same instance of QuerySet
+- Fix batch_size that was not copied when cloning a queryset object #2011
 - POTENTIAL BREAKING CHANGE: All result fields are now passed, including internal fields (_cls, _id) when using `QuerySet.as_pymongo` #1976
 - Document a BREAKING CHANGE introduced in 0.15.3 and not reported at that time (#1995)
 - Fix InvalidStringData error when using modify on a BinaryField #1127

mongoengine/queryset/base.py

@@ -757,7 +757,7 @@ def _clone_into(self, new_qs):
                       '_read_preference', '_iter', '_scalar', '_as_pymongo',
                       '_limit', '_skip', '_hint', '_auto_dereference',
                       '_search_text', 'only_fields', '_max_time_ms',
-                      '_comment')
+                      '_comment', '_batch_size')
 
         for prop in copy_props:
             val = getattr(self, prop)

tests/queryset/queryset.py

@@ -394,6 +394,16 @@ class A(Document):
         with self.assertRaises(ValueError):
             list(qs)
 
+    def test_batch_size_cloned(self):
+        class A(Document):
+            s = StringField()
+
+        # test that batch size gets cloned
+        qs = A.objects.batch_size(5)
+        self.assertEqual(qs._batch_size, 5)
+        qs_clone = qs.clone()
+        self.assertEqual(qs_clone._batch_size, 5)
+
     def test_update_write_concern(self):
         """Test that passing write_concern works"""
         self.Person.drop_collection()