code: function fetchOptions(handler, data, board, callback) {
    if (_.isEmpty(data)) {
      return callback(null, data);
    }
  
    async.eachSeries(board.selects, (item, next) => {
      if (item.select) {
        return join(handler, data, item, board.type, next);
      }
      next();
    }, err => callback(err, data));
  }
docs: 遍历所有的选择项目，获取options项的值 （与Mongo的方法相同）