where to put method that works on a model

Question

I'm working with Django.

I have a model called Agrument. Arguments have sides and owners. I have a function that returns back the side of the most recent argument of a certain user.

like obj.get_current_side(username)

I've added this to the actual Argument model like this

 def get_current_side(self, user):
        return self.argument_set.latest('pub_date').side

I am starting to think this doesn't make sense because there may not be an instance of an Argument. Is this a place I would use a class method? I thought about making a util class, but I'm thinking that it makes sense to be associated with the Argument class.

Answer 1

I think what you are looking for are model managers.

Django docs on managers with managers you can add a function to the model class instead of a model instance.

Answer 2

It would make more sense to have instance methods on the User model:

def get_current_side(self):
    try:
        return self.arguments.latest('pub_date').side
    except User.DoesNotExist, e:
        return None

You can do this by extending the User model as explained here:

• Extending the Django User model with inheritance

Edit: I'm not exactly sure which exception gets thrown.

Answer 3

This should be a method on a custom model manager:

# in models.py
class ArgumentManager(models.manager.Manager):
    def get_current_side(self, user):
        try:
            return self.filter(user=user).latest('pub_date').side
        except Argument.DoesNotExist:
            return None

class Argument(models.Model):
    # fields etc...

    objects = ArgumentManager()


# Calling:

side = Argument.objects.get_current_side(user)

Alternaticely you can extend contrib.auth.user and add get_current_size() on it. But I wouldn't mess with it until I'm very confident with Django.

BTW: Most of the code in this page is wrong; for example user variable is not used at all on the OP's snipplet.