lokiJs update array in object not persisted

Question

I use lokiJs for angualJS app. I try to update one object in collection withe an array. But data in array was lost !

$scope.seance from loki , exos is internal array
$scope.seance.exos.push($scope.exo);
try{
    $scope.seances.update($scope.seance);
}catch(err){
     console.log(err);
     alert('cant update');
}
$scope.seances.commit();
db.saveDatabase();

Thanks a lot for your help!