'Disassociate all floating IPs linked to specific port.
@param port_id: ID of the port to disassociate floating IPs.
@param do_notify: whether we should notify routers right away.
This parameter is ignored.
@return: set of router-ids that require notification updates'

@db_api.retry_if_session_inactive()
def disassociate_floatingips(self, context, port_id, do_notify=True):

    router_ids = set()
    with context.session.begin(subtransactions=True):
        for floating_ip in self._get_floatingips_by_port_id(context, port_id):
            router_ids.add(floating_ip['router_id'])
            floating_ip.update({'fixed_port_id': None, 'fixed_ip_address': None, 'router_id': None})
    return router_ids
