Normalized Collection not working with reference of child of child

Question

Angular NormalizedCollection is now working when reference of 'bookings.reservationFor.apartmentId' is passed. It was working perfect when I created a sample firebase entry and defined apartmentId as a value of 'reservationFor' directly and passed 'bookings.reservationFor' in select() parameter.

I am new to angularFire. Please let me know what is wrong with this code.

Using Firebase v2.2.9; AngularJs v1.5.6; AngularFire v1.2.0;

firebase structure

FB
    |
    --apartment
    | |
    | --apartment1
    |   |
    |   --name: "test name"
    |   --address: "test address"
    | --apartment2
    |   |
    |   --name: "test name"
    |   --address: "test address"
    |
    --booking
      |
      --"-JFZG3coHOAblHZ7XSjK"
      | |
      | --date: "booking date 1"
      | --reservatonFor: 
            |
            --apartmentId: "apartment1"
      |
      --"-KJKJASDIUOPIWE9WEeJ"
      |  |
      | --date: "booking date 2"
      | --reservatonFor: 
            |
            --apartmentId: "apartment2"
      |    
      --"-YtUTRGJLNL876F3SSwS"
      |  |
      |  --date: "booking date 3"
      | --reservatonFor: 
            |
            --apartmentId: "apartment1"
      |

Controller

function mainCtrlFunc($scope, $firebaseArray) {
var baseRef = new Firebase(firebaseUrl);

var norm = new Firebase.util.NormalizedCollection(
    [baseRef.child("booking"), "bookings"]
    [baseRef.child("apartment"), "apartments", "bookings.reservationFor.apartmentId"]
    ).select(
        "bookings.date",
        "apartments.name"
    )ref();

    $scope.bookings = $firebaseArray(norm);
}

Error: Firebase.child failed: First argument was an invalid path: "[object Object]". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]"