Adding a custom attribute to the instances

[matdrapeau]

How can we define a custom attribute for instances?

Using this route as example:

    @ItemRoute.GET
    def is_recent(self, book) -> fields.Boolean():
        return datetime.date.today().year <= book.year_published + 10

How can we define is_recent at the instances level such as:
http GET :5000/book/1 would return book attributes containing is_recent: True | False as well