code: function(keys) {
          requires(keys, 'undefined is not a valid key');
          _(arguments).forEach(keys => {
            this._select = this._select.concat(ensureArray(keys));
          });
          return this;
        }
docs: Restrict the fields of the returned AV.Objects to include only the
  provided keys.  If this is called multiple times, then all of the keys
  specified in each of the calls will be included.
  @param {String[]} keys The names of the keys to include.
  @return {AV.Query} Returns the query, so you can chain this call.