how to reference another function inside mode class

[sam]

I was tring to write a custom function inside a mode class, say X:

class X(meta.Model):
    i = meta.IntegerField()
    ...
    def foo(self):
        return bar(self.i)

If I define bar() in the same model file, django complained the
"NameError -- global name bar does not exist". How do I get around
this?

What if I want to define bar() in another file under the same
app/models directory -- how do I refer it in my model file?

Thanks,

Sam