code: function(attr) {
          if (!arguments.length) {
            return !_.isEmpty(this.changed);
          }
          return this.changed && _.has(this.changed, attr);
        }
docs: Determine if the model has changed since the last <code>"change"</code>
  event.  If you specify an attribute name, determine if that attribute
  has changed.
  @param {String} attr Optional attribute name
  @return {Boolean}