Firebase Normalized Collection and Scroll not working

Question

first the code

var baseRef = new $databaseFactory();
var childRegistration = baseRef.child("registrations/");
var childStudents = baseRef.child("students/");
$scope.scrollRef = new Firebase.util.Scroll(childRegistration,'registerDate');

var normRegisteredStudens = new Firebase.util.NormalizedCollection(
    [childStudents, "student"],
    [$scope.scrollRef, "registration"]
).select(
    "student.id",
    "student.avatarImg",
    "registration.registerDate",
    "registration.entryMethod"
).ref();

$scope.lastRegisteredStudents = $firebaseObject( normRegisteredStudens );

$scope.loadRegisteredStudents = function() {
    $scope.scrollRef.scroll.next(1);
};

data structure

"registrations" : {
"STD32159500" : {
  "entryMethod" : "web",
  "registerDate" : 1447425200913
},
"STD32159501" : {
  "entryMethod" : "web",
  "registerDate" : 1447430433895
}

"students" : {
"STD32159500" : {
  "avatarImg" : "students/default-avatar-male.png",
  "id" : "STD32159500",
},
"STD32159501" : {
  "avatarImg" : "students/default-avatar-female.png",
  "id" : "STD32159501",
}

there is no error but i still no getting one by one registration value with the function "loadRegisteredStudents", instead of that, i get all results.

i get this on load web

STD32159500:
    avatarImg: "students/default-avatar-male.png"
    entryMethod: "web"
    id: "STD32159500"
    registerDate: 1447425200913

STD32159501:
    avatarImg: "students/default-avatar-female.png"
    entryMethod: "web"
    id: "STD32159501"
    registerDate: 1447430433895