record.dart

class Record {
  final String name;
  final int votes;
  final DocumentReference reference;

  // for null safety
  // Initializer Lists constructor
  // option arguments
  // https://makicamel.hatenablog.com/entry/2019/03/14/213933
  Record.fromMap(Map<String, dynamic> map, {this.reference})
    : assert(map["name"] != null),
      assert(map["votes"] != null),
      name = map["name"],
      votes = map["votes"];

  // Initializer Lists constructor
  Record.fromSnapshot(DocumentSnapshot snapshot)
    : this.fromMap(snapshot.data, reference: snapshot.reference);

  @override
  String toString() => "Record<$name: $votes>";
}