Flutter realtime database onValue parsing

Question

I'm experimenting with NoSQL (firebase realtime database) for the first time and I'm having some issues structuring and parsing data in a Flutter app. At first I had a simple "Person" model with some attributes and all was fine but then I had to introduce an ID as node and nesting the other attributes in order to perform CRUD operations and now I'm not able to parse my "updated" 'Person' model anymore. I don't know if that's ok but, for simplicity (I know it's not a proper ID), I decided that my ID ('personName' attribute) is the name of the person so the current structure on DB is:

I'm using freezed package and PersonDto looks like this (domains methods omitted):

@freezed
class PersonDto with _$PersonDto  {
  const PersonDto ._();

  const factory PersonDto ({
    required String personName,
    required int age,
    required String genre,
    required double height,
    required String hobby,
    required double weight,
  }) = _PersonDto ;

  factory PersonDto.fromJson(Map<String, dynamic> json) =>
      _$PersonDto FromJson(json);

In the repository there's a method responsible to receive, parse and stream data from firebase. My problem is basically that I'm not able to generate a 'Person' model using the key of the node as an attribute for the 'name'. This is the closest I get :

Stream<Either<PersonFailure, List<Person>>> watchAll() async* {
yield* _firebaseDatabase
    .reference()
    .child('persons')
    .onValue
    .map((event) {
  return right<PersonFailure, List<Person>>(
      (event.snapshot.value as LinkedHashMap).values.map((personMap) {
    final json = Map<String, dynamic>.from(personMap as LinkedHashMap);
//
//this snippet works. I'm able to generate a proper 'Person' model but like this 
//there's no way to retrieve the key from event.snapshot.value
//
    json.addAll({
      'personName': 'NAME OF THE PERSON',
    });
   return PersonDto.fromJson(json).toDomain();     
  }).toList());
}).onErrorReturnWith((e) {
  print('WATCH ERROR ${e.toString()}');
  return left(const PersonFailure.unexpected());
});
}

Should be something like this, which likewise doesn't work, unfortunately:

yield* _firebaseDatabase
    .reference()
    .child('persons')
    .onValue
    .map((event) {
  (event.snapshot as LinkedHashMap).map((key, value) {
    final personName = key;
    final json = value as Map<String,dynamic>;
    json.addAll({'personName':personName});
    //
    //error: The return type 'Person' isn't a 'MapEntry<_, _>', as required by the closure's context.
    //
    return PersonDto.fromJson(json).toDomain();
  });
});

Answer 1

Just need to add a null check. like this;

.....
.onValue
.map((event) {
    List<ListsModel> _offers = <ListsModel>[];
    if(event.snapshot.value != null) {
        final _resultList =
          Map<String, dynamic>.from(e.snapshot.value as LinkedHashMap);
        for (var key in _resultList.keys) {
        Map<dynamic, dynamic> map = Map.from(_resultList[key]);
        ListsModel listsModel = ListsModel.fromMap(map);
        _offers.add(listModel);
    }
}
.....