code: function(attr) {
        this._refreshCache();
  
        var currentChanges = _.last(this._opSetQueue);
  
        if (attr) {
          return (currentChanges[attr] ? true : false);
        }
        if (!this.id) {
          return true;
        }
        if (_.keys(currentChanges).length > 0) {
          return true;
        }
        return false;
      }
docs: Returns true if this object has been modified since its last
  save/refresh.  If an attribute is specified, it returns true only if that
  particular attribute has been modified since the last save/refresh.
  @param {String} attr An attribute name (optional).
  @return {Boolean}