use flutter json annotation one model into an other model

Question

i have two models one is user model and other is milestone model. from the api i will ge a list of milestones like this with other user's data user: { first_name: "Foo", last_name: "Bar", "milestones": [ { "id": 1, "user_id": 1, "title": "first mile", "start_date": 1704455618, "end_date": 1704455618, "notified": 0 } ] }

so first i createda milestone model like this

// ignore_for_file: non_constant_identifier_names

import 'package:json_annotation/json_annotation.dart';

part 'milestone.g.dart';

@JsonSerializable()
class MileStone {
  int? id;
  int? user_id;
  String? title;
  bool? notified;
  @JsonKey(fromJson: _unixToDateTime)
  DateTime? start_date;
  @JsonKey(fromJson: _unixToDateTime)
  DateTime? end_date;
  MileStone({
    this.id,
    this.user_id,
    this.title,
    this.notified,
    this.start_date,
    this.end_date

  });

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

  Map<String, dynamic> toJson() => _$MileStoneToJson(this);

  static DateTime? _unixToDateTime(dynamic value) => value == null ? null :
  (value is int) ? DateTime.fromMillisecondsSinceEpoch(value * 1000, isUtc: true) :  (value is String) ? DateTime.parse(value) :value;
}

and then i used it in my user model like this

class User {
  String first_name;
  String? last_name;
  @JsonKey(name: 'milestones',fromJson: _milestonesList)
  List<MileStone>? milestones;

  User({
    required this.first_name,
    this.last_name,
    this.milestones,

  });

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

  Map<String, dynamic> toJson() => _$UserToJson(this);

  static List<MileStone>? _milestonesList(List<dynamic>? list) =>
      list?.map((dynamic item) => MileStone.fromJson(item as Map<String, dynamic>)).toList();

}

but i am getting null wheneven i do

if(state is UserLoggedIn){ print(state.user.milestones); }

what i am doing wrong?

i am trying to save midlestone data using milestone model in user's model.