docs: function(diff) {
          if (!diff) {
            return this.hasChanged() ? _.clone(this.changed) : false;
          }
          var changed = {};
          var old = this._previousAttributes;
          AV._objectEach(diff, function(diffVal, attr) {
            if (!_.isEqual(old[attr], diffVal)) {
              changed[attr] = diffVal;
            }
          });
          return changed;
        }