code: def contribute_to_class(self, cls, name):
          
          super(KeywordsField, self).contribute_to_class(cls, name)
          string_field_name = list(self.fields.keys())[0] % \
                              self.related_field_name
          if hasattr(cls, "search_fields") and name in cls.search_fields:
              try:
                  weight = cls.search_fields[name]
              except TypeError:
                  # search_fields is a sequence.
                  index = cls.search_fields.index(name)
                  search_fields_type = type(cls.search_fields)
                  cls.search_fields = list(cls.search_fields)
                  cls.search_fields[index] = string_field_name
                  cls.search_fields = search_fields_type(cls.search_fields)
              else:
                  del cls.search_fields[name]
                  cls.search_fields[string_field_name] = weight
docs: Swap out any reference to ``KeywordsField`` with the
          ``KEYWORDS_FIELD_string`` field in ``search_fields``.