Thread safe field value counting - django

Question

Is there any way to do something like this in django?
INSERT INTO t VALUES(1,(SELECT x.c2 FROM t x ORDER BY c2 DESC LIMIT 1)+1,1);
I have a model with many fields. And one of the field's value must be set with accordance to previous record.
Currently I am doing this by simple selecting previous record. But this is awful and not thread safe.
def save(self, *args, **kw):
if self.sn is None or self.sn == ns.DEFAULT_GIFT_SN:
q = Gift.objects.filter(company = self.company).only('id').order_by('-id')
if q.count():
last = q[0]
next_id = int(last.sn) + 1
else:
next_id = 1
self.sn = next_id
super(Gift, self).save(*args, **kw)
I want to sth. lazy like:
def save(self, *args, **kw):
if self.sn is None or self.sn == ns.DEFAULT_GIFT_SN:
self.sn = _F('SELECT x.c2 FROM t x ORDER BY c2 DESC LIMIT 1')
super(Gift, self).save(*args, **kw)
Any suggestions?
UPDATE(for Cesar):
class Gift(models.Model):
company = models.ForeignKey(Company)
certificate = ChainedForeignKey(Certificate,
chained_field = "company",
chained_model_field = "company",
show_all = False,
auto_choose = True
)
gifter = models.ForeignKey(User, null = True, blank = True)
giftant_first_name = models.CharField(_('first name'), max_length = 30, blank = True)
giftant_last_name = models.CharField(_('last name'), max_length = 30, blank = True)
giftant_email = models.EmailField(_('e-mail address'), blank = True)
giftant_mobile = models.CharField(max_length = 20, blank = True, null = True)
due_date = models.DateTimeField()
exp_date = models.DateTimeField()
sn = models.CharField(max_length = 32, default = ns.DEFAULT_GIFT_SN, editable = False)
pin_salt = models.CharField(max_length = 5, editable = False, default = gen_salt)
pin = models.CharField(max_length = 32, null = True, blank = True)
postcard = models.ForeignKey(Postcard)
state_status = models.IntegerField(choices = ns.STATE_STATUSES, default = ns.READY_STATE_STATUS)
delivery_status = models.IntegerField(choices = ns.DELIVERY_STATUSES, default = ns.WAITING_DELIVERY_STATUS)
payment_status = models.IntegerField(choices = ns.PAYMENT_STATUSES, default = ns.NOT_PAID_PAYMENT_STATUS)
usage_status = models.IntegerField(choices = ns.USAGE_STATUSES, default = ns.NOT_USED_USAGE_STATUS)
nominal = models.FloatField(default = 0)
used_money = models.FloatField(default = 0)
use_date = models.DateTimeField(blank = True, null = True)
pub_date = models.DateTimeField(auto_now_add = True)
cashier = ChainedForeignKey(CompanyUserProfile, blank = True, null = True,
chained_field = "company",
chained_model_field = "company",
show_all = False,
auto_choose = True
)
class Meta:
unique_together = ('company', 'sn',)

Answer 1

I know while is evil but you can try something like that :
sn_is_ok = False
while not sn_is_ok:
last_id = MyModel.objects.latest('id').id
try:
self.sn = last_id + 1
self.save()
Except IntegrityError:
continue
else:
sn_is_ok = True
I don't think you get more that 2 loops.

Answer 2

I guess you can do three things, from simple to outraging mad on the extremes:
Let Gift.sn be a AutoField, if you can get away without needing to increment the values separately for each company.
Use a unique_together = ('company', 'sn') constraint and update the gift instance if the uniqueness constraint fails, use the example #Stan provided, with the worst-case scenario that if you have a lot of concurrent writes only one will pass in each turn.
Write some custom SQL to obtain a lock on the Gift model table if the database supports it.