The way it works in the country/state example is by using an onchange function that returns a domain:

@api.onchange('country_id')
    def _onchange_country_id_wrapper(self):
    res = {'domain': {'state_id': []}}
            if self.country_id:
                    res['domain']['state_id'] = [('country_id', '=', self.country_id.id)]
    return res

The benefit of the onchange approach is that it allows you to select a state without a country. But it also means if you select a country, save the record, edit again and select state, it will no longer be filtered to the correct country.

If you'd prefer that the country must be selected, you can add a domain on the state field in xml like so:

 <field name="state_id" domain=[('country_id', '=', country_id)]"/>