Question

With an instance of Concert I get: unbound method do_stuff() must be called with Concert instance as first argument (got ModelBase instance instead)

models.py:

class Event(models.Model):
    def do_stuff(self):
        response self.do_specific_stuff(self)


class Concert(Event):
    def do_specific_stuff(self):
        ...

class Party(Event):
    def do_specific_stuff(self):
        ...

views:

def index(request):
    x = Concert.objects.get(name='Jack White @ Oakland')
    output = x.do_stuff()
    return HttpResponse(output)

My goal is to loop trough all the events and execute the do_specific_stuff child class method based on what kind of event it is.

Answer 1

In Django, inheritance triggers multi-table inheritance, but you don’t get the polymorphism in Python. It’s just an instance of the ORM not providing a perfect correspondence between the data schema and the object model.

In other words, when you query Event, you get back a whole bunch of Event objects, regardless of whether some of them are actually Concert or Party objects. You have to manually downcast. If an Event is a Concert, it will have an attribute called concert, which points to the corresponding Concert subclass. Likewise for Party. If it is just a normal Event, it will have neither attribute.

You could use the following property on Event to automatically downcast your object:

@property
def as_child_class(self):
    """Casts this object to its subclass, if possible"""

    if hasattr(self, 'concert'):
        return self.concert
    elif hasattr(self, 'party'):
        return self.party
    else:
        return self

Then you could do something like:

for event in Event.objects.all()
    event.as_child_class.do_specific_stuff()

Similar questions have come up before:

• Polymorphism in Django
• How do I access the child classes of an object in django without knowing the name of the child class?

And this link has some other ideas:

• http://jeffelmore.org/2010/11/11/automatic-downcasting-of-inherited-models-in-django/