Document unique de Firestore à un objet Dart

Stream<List<VehicleCommentSessionModel>> getSomeoneCommentsList(
   {@required String sellerId}) {
  return _fbd
     .collection('comments')
    .where('sellerId', isEqualTo: sellerId)
    .snapshots()
    .map((qSnap) => qSnap.docs
        .map((doc) => VehicleCommentSessionModel.fromJson(doc.data()))
        .toList());
}
Lovely Lyrebird