Override AngularFire's $save method to do an update, if a list of fields is provided.

override_save.js

var SmartSaveFactory = $FirebaseArray.$extendFactory({
  $save: function(indexOrItem, listOfFields) {
    if( !listOfFields ) {
      // do a normal save if no list of fields is provided
      return $FirebaseArray.prototype.$save.apply(this, arguments);
    }
    else {
      // this is a bit risky since we're using an internal method that could change
      // we could remove this coupling by always passing the item instead of allowing
      // an index, which would prevent relying on a private method
      var item = self._resolveItem(indexOrItem);
      var key = self.$keyAt(item);
      if( key !== null ) {
        var updateFields = pickFields(item, listOfFields);
        return self.$inst().$update(key, $firebaseUtils.toJSON(updateFields))
          .then(function(ref) {
            self.$$notify('child_changed', key);
            return ref;
          });
      }
      else {
        return $firebaseUtils.reject('Invalid record; could determine its key: '+indexOrItem);
      }
    }
  }
});

function pickFields(data, fields) {
  var out = {};
  angular.forEach(fields, function(k) {
    out[k] = data.hasOwnProperty(k)? data[k] : null;
  });
  return out;
}