How to parse List of objects from firebase realtime database Flutter

Question

I've been struggling a lot with this, not able to parse the data from my firebase realtime database using the following code. I have tried everything but I cannot do so. Below is code of necessary files. I'm using provider for state management.

My Firebase Realtime DB: Consist of 2 documents, 1 contains List of object as you can see, 2nd is just a single object.

My Provider code:

class StockProvider with ChangeNotifier {
  List<ChartHistoricalData> _histChartData = [];
  List<ChartHistoricalData> get histChartData => _histChartData;

  Future<void> fetchHistoricalStockData(ref) async {
    ref.once().then((DataSnapshot data) {
      print(data.value['SBIN_Historical']);
      _histChartData = List<ChartHistoricalData>.from(data
          .value['SBIN_Historical']
          .map((x) => ChartHistoricalData.fromJson(x as Map<String, dynamic>)));
    notifyListeners();
    });
  }
}

The widget where I call:

class _SBINChartState extends State<SBINChart> {
  final fb = FirebaseDatabase.instance;
  @override
  Widget build(BuildContext context) {
    final ref = fb.reference();
    context.read<StockProvider>().fetchHistoricalStockData(ref);
    ...

My Model Class:

class ChartHistoricalData {
  DateTime? x; //DateTime
  double? open;
  double? high;
  double? low;
  double? close;

  ChartHistoricalData(
      {required this.x,
      required this.open,
      required this.high,
      required this.low,
      required this.close});

  ChartHistoricalData.fromJson(Map<String, dynamic> json) {
    x = DateTime.parse(json['date']);
    open = json['open'];
    high = json['high'];
    low = json['low'];
    close = json['close'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = new Map<String, dynamic>();
    data['date'] = this.x;
    data['open'] = this.open;
    data['high'] = this.high;
    data['low'] = this.low;
    data['close'] = this.close;
    return data;
  }
}

I'm getting the following error:

Reloaded 1 of 620 libraries in 1,111ms.
I/flutter (32700): [{date: 2021-01-01, high: 280.0, low: 274.4, close: 279.4, open: 274.9}, {date: 2021-01-04, high: 283.9, low: 277.75, close: 281.05, open: 281.85}, {date: 2021-01-05, high: 282.45, low: 277.0, close: 281.75, open: 278.05}, {date: 2021-01-06, high: 289.15, low: 281.4, close: 285.05, open: 283.0}, {date: 2021-01-07, high: 291.8, low: 287.0, close: 287.7, open: 289.0}, {date: 2021-01-08, high: 291.4, low: 285.2, close: 286.0, open: 290.1}, {date: 2021-01-11, high: 288.2, low: 279.6, close: 282.5, open: 288.0}, {date: 2021-01-12, high: 293.85, low: 277.9, close: 292.5, open: 280.0}, {date: 2021-01-13, high: 308.0, low: 294.5, close: 306.8, open: 296.0}, {date: 2021-01-14, high: 309.25, low: 303.8, close: 307.25, open: 306.7}, {date: 2021-01-15, high: 310.9, low: 301.3, close: 303.85, open: 306.8}, {date: 2021-01-18, high: 308.65, low: 292.2, close: 294.45, open: 303.5}, {date: 2021-01-19, high: 302.5, low: 296.4, close: 298.6, open: 297.65}, {date: 2021-01-20, high: 304.7, low: 296.85, close: 302.55, open: 298.8
E/flutter (32700): [ERROR:flutter/shell/common/shell.cc(103)] Dart Unhandled Exception: type '_InternalLinkedHashMap<Object?, Object?>' is not a subtype of type 'Map<String, dynamic>' in type cast, stack trace: #0      StockProvider.fetchHistoricalStockData.<anonymous closure>.<anonymous closure>
package:algoguru/providers/stock_provider.dart:15
E/flutter (32700): #1      MappedListIterable.elementAt (dart:_internal/iterable.dart:412:31)
E/flutter (32700): #2      ListIterator.moveNext (dart:_internal/iterable.dart:341:26)
E/flutter (32700): #3      new List.from (dart:core-patch/array_patch.dart:40:17)
E/flutter (32700): #4      StockProvider.fetchHistoricalStockData.<anonymous closure>
package:algoguru/providers/stock_provider.dart:13
E/flutter (32700): <asynchronous suspension>
E/flutter (32700): #5      StockProvider.fetchHistoricalStockData
package:algoguru/providers/stock_provider.dart:11
E/flutter (32700): <asynchronous suspension>
E/flutter (32700):

If anyone could help, I would really appreciate it, it's been long debugging...

Answer 1

Okay so finally! It is not a great solution but it did work for me. I don't know and i'm not sure about the data type which firebase_database return us, but by first json encoding snapshot instance data and then json decoding it solved the data type issue. The Code:

Future<void> fetchHistoricalStockData(ref) async {
    return ref.once().then((DataSnapshot data) {
      final cleanup = jsonDecode(jsonEncode(data.value)); // THIS IS THE ANSWER
      _histChartData = List<ChartHistoricalData>.from(cleanup['SBIN_Historical']
          .map((x) => ChartHistoricalData.fromJson(x as Map<String, dynamic>)));
      notifyListeners();
    });
  }

Answer 2

You can cast the data.value to a Map<String, dynamic> and use it in the fetchHistoricalStockData method like below:

Future<void> fetchHistoricalStockData(ref) async {
    ref.once().then((DataSnapshot data) {
    Map<String, dynamic> dataValue = data.value as Map<String, dynamic>;

      _histChartData = List<ChartHistoricalData>.from(dataValue['SBIN_Historical']
          .map((x) => ChartHistoricalData.fromJson(x as Map<String, dynamic>)));
    notifyListeners();
    });
  }