code: 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;
    }
  }
docs: Get the identifier for the entity based upon its type.
  @param {String} entityType - The type of entity.
  @param {Object} entity - The entity details.
  @returns {String} The entity key.