support a negative dec operator

mongoengine/queryset/transform.py

@@ -233,8 +233,7 @@ def update(_doc_cls=None, **update):
                 # Support decrement by flipping a positive value's sign
                 # and using 'inc'
                 op = 'inc'
-                if value > 0:
-                    value = -value
+                value = -value
             elif op == 'add_to_set':
                 op = 'addToSet'
             elif op == 'set_on_insert':

tests/queryset/queryset.py

@@ -1826,6 +1826,11 @@ class QuerySetTest(unittest.TestCase):
         post.reload()
         self.assertEqual(post.hits, 10)
 
+        # Negative dec operator is equal to a positive inc operator
+        BlogPost.objects.update_one(dec__hits=-1)
+        post.reload()
+        self.assertEqual(post.hits, 11)
+
         BlogPost.objects.update(push__tags='mongo')
         post.reload()
         self.assertTrue('mongo' in post.tags)