How to apply a partial update to an object using AngularFire - angularjs

Question

The $save() in Angularfire 0.8 is confusing me.
Here's a minimal example - a snippet from my controllers.js file:
.controller('LandingPageController', ['$scope','$firebase', function($scope,$firebase) {
$scope.addNode = function() {
var FB = new Firebase('https://protodb.firebaseio.com/testrecords/');
var fbr = $firebase(FB);
fbr.$set(1,{firstname: 'James'});
}
$scope.addAttribute = function() {
var FB = new Firebase('https://protodb.firebaseio.com/testrecords/1');
var fbr = $firebase(FB).$asObject();
fbr.lastname = "Bond";
fbr.$save();
}
}])
When addNode() is called, sure enough, a node is created in my firebase:
But when addAttribute() is called, the entire record is replaced, rather than what I expected, which was for the 'lastname' attribute to be added.
I've no doubt misunderstood the docs. Can anyone help?
Update:
OK, I needed to wait until the object was loaded. It works now, after changing addAttribute to:
$scope.addAttribute = function() {
var FB = new Firebase('https://protodb.firebaseio.com/testrecords/1');
var fbr = $firebase(FB).$asObject();
fbr.$loaded().then(function() {
fbr.lastname = "Bond";
fbr.$save();
});
}

Answer 1

As you found out yourself already:
a FirebaseObject (as returned by $asObject()) does not have a $update method.
when you call $save() on a FirebaseObject before it is completely loaded, you may end up deleting other properties
To patch existing data you can:
Either wait for the entire object to be loaded (as you did in your update to the question)
Or call $firebase.$update directly
$firebase(FB).$update({ lastname: "Bond" });
This last approach has the advantage that you don't pull down the entire object, only to update a single property. Note that this is probably premature optimization in most cases, but still...