code: function(snap) {
          var newModel = Backbone.Firebase._checkId(snap);
  
          if (typeof newModel === 'object' && newModel !== null) {
            var diff = _.difference(_.keys(this.attributes), _.keys(newModel));
            _.each(diff, _.bind(function(key) {
              this.unset(key);
            }, this));
          }
  
          // check to see if it needs an id
          this._setId(snap);
  
          return newModel;
        }
docs: Unset attributes that have been deleted from the server
  by comparing the keys that have been removed.