fix BaseQuerySet.fields when mixing exclusion/inclusion with complex values like $slice

mongoengine/queryset/base.py

@@ -933,7 +933,20 @@ class BaseQuerySet(object):
             key = '.'.join(parts)
             cleaned_fields.append((key, value))
 
-        fields = sorted(cleaned_fields, key=operator.itemgetter(1))
+        # Sort fields by their values, explicitly excluded fields first, then
+        # explicitly included, and then more complicated operators such as
+        # $slice.
+        def _sort_key(field_tuple):
+            key, value = field_tuple
+            if isinstance(value, (int)):
+                return value  # 0 for exclusion, 1 for inclusion
+            else:
+                return 2  # so that complex values appear last
+
+        fields = sorted(cleaned_fields, key=_sort_key)
+
+        # Clone the queryset, group all fields by their value, convert
+        # each of them to db_fields, and set the queryset's _loaded_fields
         queryset = self.clone()
         for value, group in itertools.groupby(fields, lambda x: x[1]):
             fields = [field for field, value in group]

tests/queryset/field_list.py

@@ -141,6 +141,16 @@ class OnlyExcludeAllTest(unittest.TestCase):
         self.assertEqual(qs._loaded_fields.as_dict(),
                          {'b': {'$slice': 5}})
 
+    def test_mix_slice_with_other_fields(self):
+        class MyDoc(Document):
+            a = ListField()
+            b = ListField()
+            c = ListField()
+
+        qs = MyDoc.objects.fields(a=1, b=0, slice__c=2)
+        self.assertEqual(qs._loaded_fields.as_dict(),
+                         {'c': {'$slice': 2}, 'a': 1})
+
     def test_only(self):
         """Ensure that QuerySet.only only returns the requested fields.
         """