How can I add a support of for Custom Types within Objectbox?

How can I add a support of for Custom Types within Objectbox?

Example:

 class ShopListData {
  int id;
  int amount;
  bool done;
  ProductData productData;

  ShopListData(
      {required this.id,
      required this.amount,
      required this.done,
      required this.productData});
}

class ProductData {
  String? name;
  String? brand;
  String? ingredients;
  String? quantity;

  ProductData({
    this.name,
    this.brand,
    this.ingredients,
    this.quantity,
  });
}

Where I get the error:

cannot use the default constructor of 'ShopListData': don't know how to initialize param productData - no such property.

I found out that one possible solution is to use relations, but since I have always 1:1 relation I think I don't need that option.

Instead I would like to store data as defined by ShopListData Class within the database. I read the custom type documentation. However I don't understand what is needed to add support/converter for the ProductData type. Who has an idea and could provide an example of how to add such support/converter?

I've edited your code like this:

@Entity()
class ShopListData {
  int id;
  int amount;
  bool done;
  ProductData productData;

  ShopListData({required this.id, required this.amount, required this.done, required this.productData});

  String get dbProductData => jsonEncode(productData.toJson());

  set dbProductData(String value) => productData = ProductData.fromJson(jsonDecode(value));
}

@Entity()
@JsonSerializable()
class ProductData {
  String? name;
  String? brand;
  String? ingredients;
  String? quantity;

  ProductData({this.name, this.brand, this.ingredients, this.quantity});

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

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

Since ProductData is a custom type for objectbox and should be mapped into a primitive type, I preferred stroring your ProductData model as a String in objectbox and converting it to a model again once it is taken form the db.

Ref: https://docs.objectbox.io/advanced/custom-types#convert-annotation-and-property-converter