The SOA record of the zone is retrieved and the minimum TTL is extracted
from the ``content`` field.
- If a SOA record does not exist for this zone, None is returned.
+ If a SOA record does not exist for this zone, PDNS_DEFAULT_RR_TTL
+ is returned from the settings..
Record = cache.get_model('powerdns_manager', 'Record')
soa_rr = Record.objects.get(domain=self, type='SOA')