docs: def relation_factory(relation_name):
      """Get the RelationFactory for the given relation name.
  
      Looks for a RelationFactory in the first file matching:
      ``$CHARM_DIR/hooks/relations/{interface}/{provides,requires,peer}.py``
      """
      role, interface = hookenv.relation_to_role_and_interface(relation_name)
      if not (role and interface):
          hookenv.log('Unable to determine role and interface for relation '
                      '{}'.format(relation_name), hookenv.ERROR)
          return None
      return _find_relation_factory(_relation_module(role, interface))