django: Downcasting automatically in multitable inheritance/polymorphic tables

I have something like this:

class Base(Model):
     ...
     def downcast(self):
         try:
             return self.childa
         except:
             pass
         try:
             return self.childb
         except:
             pass
         return self


class ChildA(Base):
     ....

class ChildB(Base):
     ....

class Foo(Model):
     child = ForeignKey(Base)

Whenever I have a Foo object, the child foreignkey is always an instance of Base - my understanding is that's how Django works. For now I have added a downcast() method to Base (see above). I don't mind hardcoding the possible derived types.

What I would like is to somehow centralize that downcast automatically in Foo. I added this multi-table inheritance to existing code and I keep finding instances where the code really needs it downcast -- so I have to then manually downcast it locally in the code.

I was using the django-polymorphic package, but it is giving me some side effects I don't know how/nor want to deal with (like I can't delete rows - got some error about opts.pk being None deep in queryset code.)

So I've wondered -- would putting something in __init__() (after calling the base class init) be ok? Are there side effects I'm not thinking of? This seems like it could be a problem when creating new instances from scratch.

def __init__(*args, **kwargs):
    super(Base, self).__init__(*arg, **kwargs)
    self.child = self.child.downcast()

Should I just rename child?

class Foo(Model):
    child_poly = ForeignKey(Base)  # was child
    @property
    def child(self):
        return self.child_poly.downcast()

This could be a problem when creating Foo() from scratch. I can't say Foo(child=c).

Is there a better approach? Not looking for a generic polymorphic solution/mixin -- not after trying to debug django and finding that removing django-polymorphic fixed the deletion issue.

In the end, I went back to django-polymorphic and haven't had the issue I was having before again.