How do you get angular to work with async requests?

gistfile1.txt

%h2 Registrations
.row{:"ng-controller" => "RegistrationsCtrl"}
  .span6
    %ul.players
      %li{:"ng-repeat" => "person in registrations"} 
        {{person.name}}
        
  .span6
    .player-info


%script{:type => "text/javascript"}
  :plain
    
    function RegistrationsCtrl($scope) {
      var registration_list = new Firebase('https://lusl-dev.firebaseIO.com/summer_league_2013/registrations');
      registration_list.on('child_added', function(child) {
        var reg = child.val();
        $scope.registrations.push(reg);
      });
    };