how to set default value to nested JSON objects?

[Qwampa]

I am trying to figure out how to properly set the default value to nested JSON fields.
I am trying to avoid null values for missing fields in the response json.

I have tried this:

@JsonSerializable(explicitToJson: true)
class LoginResponse {
  @JsonKey(defaultValue: false)
  bool success;
  @JsonKey(defaultValue: new UserDataModel())
  UserDataModel seeker;
  @JsonKey(defaultValue: 0)
  int timestamp;

but the syntax is not correct as the value must be a constant.

I have also tried this:

@JsonSerializable(explicitToJson: true)
class LoginResponse {
  @JsonKey(defaultValue: false)
  bool success;
  @JsonKey(disallowNullValue: true, defaultValue: {"seeker": {"id": "", "name": "", "email": "",}})
  UserDataModel seeker;
  @JsonKey(defaultValue: 0)
  int timestamp;
  

Generated code:

LoginResponse _$LoginResponseFromJson(Map<String, dynamic> json) {
$checkKeys(json, disallowNullValues: const ['seeker']);
return LoginResponse(
  success: json['success'] as bool ?? false,
  seeker: json['seeker'] == null
      ? null
      : UserDataModel.fromJson(json['seeker'] as Map<String, dynamic>) ??
          {
            'seeker': {
              'id': '',
              'name': '',
              'email': ''
            }
          },
  timestamp: json['timestamp'] as int ?? 0,
);
}

but when the response does not include the field, it is still set to null.
Any help would be appreciated.

[pierre-gancel]

Here is out of jail card :

UserDataModel _$UserDataModel(Map<String, dynamic> json) {
  if (json != null) {
    return UserDataModel(
      personId: json['userDataModelId'] as int ?? 0,
    );
  } else {
    return (UserDataModel(
      personId: 0,
   ));
  }
}

[Dohmanlechx]

This is in demand for us too. We should be able to use an own object as defaultValue, like @JsonKey(defaultValue: PriceData()), so this:

    basePrice: json['basePrice'] == null
        ? null
        : PriceData.fromJson(json['basePrice'] as Map<String, dynamic>),

should be changed into this:

    basePrice: json['basePrice'] == null
        ? PriceData()
        : PriceData.fromJson(json['basePrice'] as Map<String, dynamic>),

[kevmoo]

This gets a bit complicated.

We could add in logic to support the encoded version of a type in defaultsTo (or add another configuration option). Then we'd pass that in as PriceData.fromJson(json['price] as Map<String, dynamic>? ?? const {...}

But that gets super complicated!

I'm going to suggest that folks just create a fromJson converter here. Take in Map<String, dynamic>? and handle returning whatever you like in the null case.

I don't think the added complexity here is worth it!

[kevmoo]

#849 is the issue to track documening this better!

[pierre-gancel]

Hi @kevmoo,
Thanks for replying on this,
Could you please give an example of :

just create a fromJson converter here

[mrruby]

Any update on this, have you found any workaround?

[kopianan]

any update ?

[kevmoo]

Add a constructor parameter for the object with a default value. That'll work!

[parisa7103]

Add a constructor parameter for the object with a default value. That'll work!
please fix the bug:
When we set default value for parameters, parameters must be nullable and there is no choice except for adding default
values again in constructor.

[PiN73]

Starting from json_serializable 5, generated fromJson uses default parameters from constructor, so you can do this

@JsonSerializable()
class Cart {
  final List<int> items;

  const Cart({
    this.items = const [],
  });

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

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

@JsonSerializable()
class User {
  final Cart cart;

  const User({
    this.cart = const Cart(),
  });

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

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

Then Cart.fromJson({}).items or User.fromJson({}).cart.items will give you []