docs: function _getEntityKey(entityType, entity) {
    switch (entityType) {
      case 'remote-application':
      case 'application':
      case 'unit':
        return entity.name;
      case 'machine':
      case 'relation':
        return entity.id;
      case 'annotation':
        return entity.tag;
      default:
        // This is an unknown entity type so ignore it as we don't know how to
        // handle it.
        return null;
    }
  }