with_auto_parsing.dart

@JsonSerializable(explicitToJson: true)
class Outlet {
  int id;
  String name;
  String code;
  String address;
  @JsonKey(name: 'mobile_logo')
  String logo;
  @JsonKey(name: 'company_id')
  int companyId;
  @JsonKey(name: 'food_items')
  List<Map<String,dynamic>> foodItems;
  @JsonKey(name: 'food_ids')
  List foodIds;

  Outlet();

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

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

without_auto_parsing.dart

// this class has only 8 properties
// two of which are int & 2 are list

class Outlet {
  int id;
  String name;
  String code;
  String address;
  String logo;
  int companyId;
  List<Map<String, dynamic>> foodItems;
  List<int> foodIds;

  Outlet();


  Outlet fromJson(Map<String, dynamic> json) {
    var outlet =  Outlet();
    if(json['id'] != null && json['id'] == 'String'){
      outlet.id = int.tryParse(json['id']) ?? json['id'];
    }
    if(json['company_id'] != null && json['company_id'] == 'String'){
      outlet.companyId = int.tryParse(json['company_id']) ?? json['company_id'];
    }

    
    if(json['food_items'] != null && json['food_items'] is String){
      List _foodItems = jsonDecode(json['food_items']);
      outlet.foodItems = List<Map<String,dynamic>>.from(_foodItems.map((e) => jsonDecode(e)).toList());
      //
      // If this food item contains same kind of properties this this
      // entire structure will be looped followed within that too to parse correct
      // data type depending on how many nesting levels required
      //
    }
    // And in some places we have list encoded as string, so we have to find those places and add another 
    // level of jsonDecode

    if(json['food_ids'] != null && json['food_ids'] is String){
      List<String> _foodIds = jsonDecode(json['food_ids']);
      foodIds = List<int>.from(_foodIds.map((e) => int.tryParse(e) ?? 0).toList());
    }

      name = json['name'];
      code = json['code'];
      address = json['address'];
      logo = json['mobile_logo'];

    return outlet;
  }

  Map<String, dynamic> toJson(Outlet instance) {
    Map<String, dynamic> _temp = {};
    _temp['id'] = id;
    _temp['name'] = name;
    _temp['code'] = code;
    _temp['address'] = address;
    _temp['mobile_logo'] = logo;
    _temp['company_id'] = companyId;
    _temp['food_ids'] = jsonEncode(foodIds);
    var _foodItems = foodItems;
    _foodItems.map((e) => jsonEncode(e));
    // repeat the nested values in each model until it covers all sub levels
    _temp['food_items'] = jsonEncode(_foodItems);
    return _temp;
  }

}