Dart json序列化，如何处理来自mongodb的_id在Dart中是私有的？

dependencies:
  json_annotation: ^4.0.0
  json_serializable: ^4.1.0

import 'package:json_annotation/json_annotation.dart';

part 'model.g.dart';

@JsonSerializable()
class MyModel {
  @JsonKey(name: '_id')
  String id;

  // 其他字段...

  MyModel(this.id);

  factory MyModel.fromJson(Map<String, dynamic> json) => _$MyModelFromJson(json);
  Map<String, dynamic> toJson() => _$MyModelToJson(this);
}

flutter pub run build_runner build

import 'dart:convert';

// ...

// 反序列化
String jsonString = '{"_id": "12345", "otherField": "value"}';
MyModel model = MyModel.fromJson(json.decode(jsonString));

// 序列化
String serializedJson = json.encode(model.toJson());