fixed case where already nested fields (without getField) would not get converted

Author: Jibola

django_mongodb_backend/query_conversion/expression_converters.py

@@ -11,8 +11,8 @@ def is_simple_field_name(cls, field_name):
  isinstance(field_name, str)
  and field_name != ""
  and field_name.startswith("$")
- # Special case for _ separated field names
- and field_name[1:].replace("_", "").isalnum()
+ # Case for catching variables
+ and not field_name.startswith("$$")
  )
 
  @classmethod