fix JSONField negative indexing

Author: timgraham

django_mongodb_backend/fields/json.py

@@ -24,7 +24,7 @@
 def valid_path_key_name(key_name):
     # A lookup can use path syntax (field.subfield) unless it contains a dollar
     # sign or period.
-    return not any(char in key_name for char in ("$", "."))
+    return not key_name.startswith("-") and not any(char in key_name for char in ("$", "."))
 
 
 def build_json_mql_path(lhs, key_transforms, as_expr=False):
@@ -36,7 +36,12 @@ def build_json_mql_path(lhs, key_transforms, as_expr=False):
         get_field = {"$getField": {"input": result, "field": key}}
         # Handle array indexing if the key is a digit. If key is something
         # like '001', it's not an array index despite isdigit() returning True.
-        if key.isdigit() and str(int(key)) == key:
+        try:
+            int(key)
+            is_digit = str(int(key)) == key
+        except ValueError:
+            is_digit = False
+        if is_digit:
             result = {
                 "$cond": {
                     "if": {"$isArray": result},