Flutter: json_serializable ignore nullable fields instead of throwing an error

Question

Suppose there are two models User and City

@JsonSerializable()
class User {
    int id;
    String name;
    City? city;
    List<Map<String, City>>? listMapCity;

}

@JsonSerializable()
class City {
   int id;
   String name;
}

Now suppose during API call, we've got a user model but in the city object model, we only get id not name. Something like this

{
    "id": 5,
    "name": "Matthew",
    "city": {
        "id": 12
    }
}

But due to the default nature of json_serializable and json_annotation. This JSON is not mapped to the User model, during mapping, it throws the exception.
type Null is not a subtype of type String. (because here name key is missing in city object)

But as we already declared in the User object that City is optional, I wanted that it should parse the User JSON with city and listMapCity to be null.

Any help or solution would be really appreciated, Thank you

What Dart version are you using? If you are using Dart 2.12+, then you should mark the `name` field in City object as nullable - `String? name`. — mkobuolys, May 23 '21 at 10:49
I'm using Dart 2.13 but I don't want the name to be optional if the city object is coming then the id and name both should present. — Himanshu Arora, May 23 '21 at 11:08
Currently in iOS, I'm using this library https://github.com/JohnSundell/Unbox, this work just like I explained. — Himanshu Arora, May 23 '21 at 11:10
What do you want to do if only id is present and name is not there ? — Nisanth Reddy, May 23 '21 at 12:15
Currently json_serializable throws the error, is there any way to just map the null instead of throws an error? — Himanshu Arora, May 23 '21 at 12:36
If you mark the field as nullable with String? name you get the result you want, if you don't you would get an error anyway because you assign null to a non nullable field — alex, May 26 '21 at 16:41

score 9 · Answer 1 · answered May 06 '22 at 21:15

9

You need to set the includeIfNull flag to false to have the autogenerated code handle nulls correctly.

@JsonSerializable(includeIfNull: false)

The property should be declared with a ? as per your example.

answered May 06 '22 at 21:15

sweetfa

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score 2 · Answer 2 · answered Nov 17 '21 at 04:09

You need to have a default constructor on your JsonSerializable User class. Then, if name should be nullable, declare it with a nullable String? name;

Here's the updated User class.

import 'package:json_annotation/json_annotation.dart';

part 'user.g.dart';

@JsonSerializable()
class User {
  int id;
  String name;
  City? city;
  List<Map<String, City>>? listMapCity;

  User({required this.id, required this.name, this.city, this.listMapCity});

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

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

@JsonSerializable()
class City {
  int id;
  String name;
  City({required this.id, required this.name});
}

Flutter: json_serializable ignore nullable fields instead of throwing an error

2 Answers2