def parse_query_string(self, params):
        """
        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.
        """
        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