Django ModelChoiceField - use something other than id?

Question

Say I have an address table and it has a postal_code field -- ModelChoiceField does not allow me to use something other than PKs to validate existence

Answer 1

If postal_code is a foreign key to a PostalCode model that contains valid postal codes I would just use use a CharField and then do a clean like you suggested. My clean method would look like this:

def clean_postal_code(self):
    try:
        code = PostalCode.objects.get(code_field=self.data['postal_code'])
    except:
        raise forms.ValidationError("Please enter a valid postal code")
    return code