use promise to get only portion of ajax response

UPDATE: Ah, now I think I got what you're after: You want to have the empty reference (placeholder) for a field of the returned object, in a call like this:

var User = $resource('/user/:userId', {userId:'@id'});

Am I right?

If so - I think the easiest way is simply to bind to the object's field (aka {{user.myvar}}).

---

Well, the success function is called only once (and if) the data has been received successfully. The function is called with the fetched data given to it as an argument and you can access it no matter its structure. You can reuse a function instead of defining one inline, of course.

So instead of this (example from Angular docs):

$http({method: 'GET', url: '/someUrl'}).
  success(function(data, status, headers, config) {
    // this callback will be called asynchronously
    // when the response is available
  }).
  error(function(data, status, headers, config) {
    // called asynchronously if an error occurs
    // or server returns response with an error status.
  });

You can use:

function myHandler(data) {
     // Do something with data.myvar ...
}

$http({method: 'GET', url: '/someUrl'}).success(myHandler);

Hopefully this helps, but very possibly I did not get what exactly you're after :) Let me know.

Relevant documentation.

You need to call get() or query() on your resource to make an actual AJAX request. If you don't want to assign the full result of the server API directly to some ($scope) variable, then you have to write the success function.

var SomeResource = $resource('/blah/blah');
var theResource = SomeResource.get({optional params here}, function() {
    $scope.myVar = theResource.myVar;
}

or

var SomeResource = $resource('/blah/blah');
SomeResource.get({optional params here}, function(theResource) {
    $scope.myVar = theResource.myVar;
}

Or do what @Chasseur recommends -- assign the full result then bind to the appropriate field in your view:

var SomeResource = $resource('/blah/blah');
$scope.theResource = SomeResource.get({optional params here});

Then in HTML use {{theResource.myVar}}.

Also, $resource, unlike $http, does not return a promise. Update: in newer versions of Angular, $resource now exposes a promise.