Question

I use SQLAlchemy to define a User class.

if an user is not logged on my website, I want to identify him as an AnonymousUser. I want my AnonymousUser to be a subclass of my normal User class, with special method (eg. is_anonymous(self): return true).

Now, there is an anonymous user that I created initially in my database, with (say) id = 0.

I would like to define AnonymousUser as a subclass of User, where the constructor only returns the anonymous user :

class AnonymousUser(User):
    def __init__(self):
        anonymous_user = User.get_by_id(0)
        self.username = anonymous_user.username
        self.id = anonymous_user.id
        ...

but when I create an AnonymousUser, it does not understand that I don’t want a new User, but only to create an existing User, and it commits the creation of a new User to the session, and since the id has been set to the existing anonymous_user id it crashes because of the id unicity condition

How can I tell SQLAlchemy to create a User object using an existing id?

Is there a more elegant way to do what I want?

I know that I could simply define an anonymous_user by using User.get_by_id(0), but then I would have to add an is_anonymous field to my object and have a variable check each time I check whether my user is anonymous (rather than having two distinct classes, one with a trivial implementation is_anonymous: return False and the other using is_anonymous: return True

Answer 1

A quick suggestion. You can implement your is_anonymous method on the User class like this:

def is_anonymous(self):
    return self.id == 0