How can I get only the properties that are custom attributes on the model itself?

I have a custom model attr (similar to what ships with ember-data)

var attr = function() {
    var value = '';
    return function(key, val) {
        if (arguments.length === 2) {
            value = val;
        }
        return value;
    }.property()
};

I annotate my model properties with it like so

var Foo = Ember.Object.extend({
    firstName: attr()
});

What I'm looking for is a way to get "only" these attrs when I loop through the "keys" on this instance (example -in my rollback method I want to reset each of these - but no other keys should be reset)

var Foo = Ember.Object.extend({
    firstName: attr(),
    rollback: function() {
        for(var key in this){
            this.set(key, "some value here");
        }
    }
});

It it possible to make this attr special in some way so I can loop over only those attribues on the model?