Ionic and ng-class problem

ionic-v1
1

I have a DataBase on firebase like this one:

{
Dogs {
   Dog1{
       name: 'name1',
       breed: 'breed1',
       sold: true
          }
      }
}

and I have a set of cards on Ionic showing the information of every dog.

I would like to be able to change the css of the cards when the value sold is true like changing its background color or setting a text.

It is possible using firebase?

This is my try:

Controller:

$scope.soldClass = [];

var firebaseObj = new Firebase("https://<MY-APP>.firebaseio.com/Dogs");
 firebaseObj.orderByChild("sold").on("child_added", function(snapshot) {
        console.log(snapshot.key() + " was " + snapshot.val().sold + " sold");
        $scope.soldClass = snapshot.val().sold;
        return $scope.soldClass;

        });

and my Div:

 <div class="item item-avatar" ng-class="{sold: soldClass == 'true'}"> 
CARD
</div>

The problem is that it applies the class to all the cards even when on the database the sold value is false.