Automatically call .toJson() of children objects in _$<NameOfObject>ToJson(<Object>)

[nicholaschiang]

Why doesn't the json_serializable package automatically call toJson() for child objects?

For example, in message.dart, I've got:

import 'package:json_annotation/json_annotation.dart';
import 'package:meta/meta.dart';

import 'user.dart';

part 'message.g.dart';

@JsonSerializable()
class Message {
  const Message({
    @required this.id,
    @required this.text,
    @required this.sentBy,
    @required this.timestamp,
  })  : assert(id != null),
        assert(text != null),
        assert(sentBy != null),
        assert(timestamp != null);

  final String id;
  final String text;
  final User sentBy;
  final DateTime timestamp;

  @override
  String toString() => "Message from ${sentBy.name} ($id)";

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

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

And that User object is also @JsonSerializable() as seen in user.dart:

import 'package:json_annotation/json_annotation.dart';
import 'package:meta/meta.dart';

part 'user.g.dart';

@JsonSerializable()
class User {
  final String id;
  final String name;
  final String username;
  final String photo;
  bool active;

  User({
    @required this.id,
    @required this.name,
    @required this.photo,
    @required this.username,
    @required this.active,
  })  : assert(id != null),
        assert(name != null),
        assert(photo != null),
        assert(username != null),
        assert(active != null);

  String get firstName => name.split(' ')[0];
  String get lastName => name.split(' ')[name.split(' ').length - 1];

  @override
  String toString() => "$name a.k.a. $username ($id)";

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

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

Except that the generated message.g.dart (by running $ flutter pub run build_runner build) doesn't automatically call that toJson() method (which to me seems strange):

Map<String, dynamic> _$MessageToJson(Message instance) => <String, dynamic>{
      'id': instance.id,
      'text': instance.text,
      'sentBy': instance.sentBy,
      'timestamp': instance.timestamp?.toIso8601String()
    };

Any suggestions (besides manually editing the _$MessageToJson method)?

[nicholaschiang]

Current workaround is just to manually edit message.g.dart to look like this:

Map<String, dynamic> _$MessageToJson(Message instance) => <String, dynamic>{
      'id': instance.id,
      'text': instance.text,
      'sentBy': instance.sentBy.toJson(),
      'timestamp': instance.timestamp?.toIso8601String()
    };

Not the best solution, but it works for now.

[kevmoo]

This is by design. The JSON encoding logic in dart:convert calls toJson for you, which means calls to toJson are lazy.

You can override this by using https://pub.dev/documentation/json_annotation/latest/json_annotation/JsonSerializable/explicitToJson.html

[masus04]

Calling toJson on JsonSerializable types automatically would simplify use a lot 👍