Use dart json_serializable to map an object field to JSON flat keys (one to many mapping)

[luissalgadofreire]

I have a dart object that includes a field of type Money, which itself is composed of amount and currency:

@JsonSerializable()
class Account {

  final String id;
  final String type;
  final String subtype;
  final String origin;
  final String name;
  final String status;
  final String currency;
  final Money balance; <== value object
  ...
}

Money looks something like this:

class Money {
  final int amount;
  final String currency;

  const Money(this.amount, this.currency);
  ...
}

The above is to be mapped for use by sqflite, therefore the target JSON must be a flat JSON, like:

{
  "id": String,
  "type": String,
  "subtype": String,
  "origin": String,
  "name": String,
  "status": String,
  "currency": String,
  "balanceAmount": int;      <== value object
  "balanceCurrency": String; <== value object
}

I understand I can use JsonKey.readValue to extract a composite object from the entire JSON object prior to decoding.

But how can I do the opposite? Grab the two values from the Money instance and map them to balanceAmount and balanceCurrency?

Nothing in the search I did on json_serializable api docs, GitHub issues, or StackOverflow appears to respond to this in particular: how to map one field to two (or more) target keys?

[mbaguszulmi]

Maybe json_seriazable doesn't have this functionality yet for now. So in manual approach, it could be like this

@JsonSeriazable()
class Account {
  // properties...

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

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

  // add this method
  Map<String, dynamic> toFlatJson() {
    final currentJson = toJson();
    final newJson = {...currentJson};

    // flattening 'balance' value
    currentJson['balance']!.forEach((k, v) {
      newJson[k] = v;
    });

    // remove 'balance' value from newJson
    newJson.remove('balance');

    return newJson;
  }
}

Note:

Well, I didn't test this yet. So you might manipulate this code snippet based on your own problems.

[PackRuble]

At the moment I also have not found any other way than to use readValue and change the toJson method myself. I'll show how this can be done using freezed:

|-- freezed 2.5.2
|   |-- source_gen 1.5.0
|-- freezed_annotation 2.4.1

|-- json_annotation 4.9.0
|-- json_serializable 6.8.0

The models look like this (Pay attention to Account.toJson and the passJson function, which simply passes raw json):

Object passJson(Map json, _) => json;

@freezed
class Account with _$Account {
  @JsonSerializable(explicitToJson: true)
  const factory Account({
    @JsonKey(name: 'status') required String status,
    @JsonKey(name: '_balance', readValue: passJson) required Money balance,
  }) = _Account;

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

  const Account._();

  @override
  Map<String, dynamic> toJson() => toJson()
    ..addAll(balance.toJson())
    ..remove('_balance');

  Map<String, dynamic> toFlatJson() => toJson()
    ..addAll(balance.toJson())
    ..remove('_balance');
}

@freezed
class Money with _$Money {
  const factory Money({
    @JsonKey(name: 'balanceCurrency') required String currency,
    @JsonKey(name: 'balanceAmount') required int amount,
  }) = _Money;

  factory Money.fromJson(Map<String, dynamic> json) => _$MoneyFromJson(json);
}

Then the generated files look like this:

_$AccountImpl _$$AccountImplFromJson(Map<String, dynamic> json) =>
    _$AccountImpl(
      status: json['status'] as String,
      balance:
          Money.fromJson(passJson(json, '_balance') as Map<String, dynamic>),
    );

Map<String, dynamic> _$$AccountImplToJson(_$AccountImpl instance) =>
    <String, dynamic>{
      'status': instance.status,
      '_balance': instance.balance.toJson(),
    };

_$MoneyImpl _$$MoneyImplFromJson(Map<String, dynamic> json) => _$MoneyImpl(
      currency: json['balanceCurrency'] as String,
      amount: (json['balanceAmount'] as num).toInt(),
    );

Map<String, dynamic> _$$MoneyImplToJson(_$MoneyImpl instance) =>
    <String, dynamic>{
      'balanceCurrency': instance.currency,
      'balanceAmount': instance.amount,
    };

This is still not as convenient as some kind of “magic short annotation”, but it allows you to complete the task using flat json.

---

Update

Unfortunately, the method above doesn't work because the toJson method doesn't work as expected (the overridden toJson method does not run our code):

• Can't create custom method toJson · Issue #697 · rrousselGit/freezed

So I added const Account._(); to the example above and defined toFlatJson. Then:

main() {
  final json = {
    'status': 'receipted',
    'balanceCurrency': '1.0',
    'balanceAmount': 500,
  };

  final account = Account.fromJson(json);
  print(account.toJson()); // {status: receipted, _balance: {balanceCurrency: 1.0, balanceAmount: 500}}
  print(account.toFlatJson()); // {status: receipted, balanceCurrency: 1.0, balanceAmount: 500}
}