Stores the keywords as a single string for searching.

def related_items_changed(self, instance, related_manager):
        """
        Stores the keywords as a single string for searching.
        """
        assigned = related_manager.select_related("keyword")
        keywords = " ".join([str(a.keyword) for a in assigned])
        string_field_name = list(self.fields.keys())[0] % \
                            self.related_field_name
        if getattr(instance, string_field_name) != keywords:
            setattr(instance, string_field_name, keywords)
            instance.save()