def METHOD_NAME(self, *args, **kw):
    super(Subdivisions, self).METHOD_NAME(*args, **kw)
    # Add index for the country code.
    self.indices["country_code"] = {}
    for subdivision in self:
        divs = self.indices["country_code"].setdefault(
            subdivision.country_code.lower(), set()
        )
        divs.add(subdivision)