docs: function (a, b, expression) {
              var valueA = this._getValueForItemField(a, expression.field);
              var valueB = this._getValueForItemField(b, expression.field);
              if (valueA > valueB) {
                  return expression.direction;
              } else if (valueA < valueB) {
                  return -1 * expression.direction;
              } else {
                  return 0;
              }
          }