From 6a4c342e45fbd556dc1938923b7bed17031c79c7 Mon Sep 17 00:00:00 2001
From: Dmitry Voronenkov <divomen@gmail.com>
Date: Tue, 18 Jun 2019 16:13:29 +0300
Subject: [PATCH] Supported updates of an array by negative index

---
 mongoengine/base/datastructures.py | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/mongoengine/base/datastructures.py b/mongoengine/base/datastructures.py
index fafc08b7..9307556f 100644
--- a/mongoengine/base/datastructures.py
+++ b/mongoengine/base/datastructures.py
@@ -108,6 +108,9 @@ class BaseList(list):
         super(BaseList, self).__init__(list_items)
 
     def __getitem__(self, key):
+        # change index to positive value because MongoDB does not support negative one
+        if isinstance(key, int) and key < 0:
+            key = len(self) + key
         value = super(BaseList, self).__getitem__(key)
 
         if isinstance(key, slice):