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

Answer 1

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});
}

Answer 2

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.