code: def parse_query_string(self, params):
          
          results = {}
  
          for key, val in params.items():
              lookup_len = len(self.query_string_lookup) + 1
  
              if key[0:lookup_len] == '{}['.format(self.query_string_lookup) and key[-1] == ']':
                  results[key[lookup_len:-1]] = val if val.lower() != 'null' else None
  
          return results
docs: Override this method if you need to support query string filter keys other than those in the
          format of ``key[field_name]=val``. Maps query string values == 'null' to ``None``.
  
          :param params: The query string parameters from ``request.params``.
          :return: Dictionary.