code: function _getArray(elem, field) {
          var f = elem._id + "." + field;
          if (!_currentArray[f]) {
              _currentArray[f] = _.clone(elem[field]);
          }
          return _currentArray[f];
      }
docs: Return a copied array of array field of a particular element.
  It could be changed by `fieldInsert`, `fieldRemove`.
  The reason why using this is index value maybe incorrect when performing `fieldInsert` multiple times.
  
  @private
  @param {Element} elem An element
  @param {string} field Array field name
  @return {number}