docs: def register(self, entity, url_func):
          """Associate a `url_func` with entity's type.
  
          :param:entity: an :class:`abilian.core.extensions.db.Model` class or
          instance.
  
          :param:url_func: any callable that accepts an entity instance and
          return an url for it.
          """
          if not inspect.isclass(entity):
              entity = entity.__class__
          assert issubclass(entity, db.Model)
          self._map[entity.entity_type] = url_func