code: function(remoteAttributes, localAttributes) {
            var updateAttributes = {};
  
            var union = _.union(_.keys(remoteAttributes), _.keys(localAttributes));
  
            _.each(union, function(key) {
              if (!_.has(localAttributes, key)) {
                updateAttributes[key] = null;
              } else if (localAttributes[key] != remoteAttributes[key]) {
                updateAttributes[key] = localAttributes[key];
              }
            });
  
            return updateAttributes;
          }
docs: set the attributes to be updated to Firebase set any removed attributes to null so that Firebase removes them