[Flutter] 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?

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