code: function onlyOneFieldAttribute(fieldsList, keyword) {
    if (fieldsList.length > 1) {
      return Bluebird.reject(new BadRequestError(`"${keyword}" can contain only one attribute`));
    }
  
    return Bluebird.resolve(fieldsList);
  }
docs: Verifies that "filter" contains only 1 field
  @param {Array} fieldsList
  @param {string} keyword
  @returns {Promise} Promise resolving to the provided fields list