'Our best approximation of how to map facilities to zones'

@classmethod
def from_zone(cls, zone):

    facilities = set(Facility.objects.filter(zone_fallback=zone))
    for device_zone in DeviceZone.objects.filter(zone=zone):
        device = device_zone.device
        facilities = facilities.union(set(Facility.objects.filter(signed_by=device)))
    return facilities
