code: def relation_path_for(from_name, to_name, identifier_type, identifier_key=None):
      
      return "/{}/<{}:{}>/{}".format(
          name_for(from_name),
          identifier_type,
          identifier_key or "{}_id".format(name_for(from_name)),
          name_for(to_name),
      )
docs: Get a path relating a thing to another.