Hello,
I have a question about transactions and thread safe applications.
I have an invoice app, where I have a model like this:
class Counter(models.Model):
client = models.ForeignKey(Client)
number = models.CharField(max_length=10, db_index=True)
class Meta:
unique_together = (("client", "number"),)
Where number is the number of the invoice: "2010/321"
What I need is to create a view that takes the last invoice number of
a client, parse it and add 1. ie: "2010/321" -> "2010/322"
I have something like this:
@transaction.commit_on_success
def increment_counter(name, client):
counter = Counter.objects.filter(client=client).order_by('-id')[0]
#Step 2: Parse format and Increment the counter
....
newcounter = "2010/322"
counter.count = newcounter
#Step 3: Save the new counter
counter.save()
I don't know if this is the best approach.
Imagine that when the first thread (thread1) is in step2, the second
thread (thread2) just inserted a new value in the database, so the
counter is already on "2010/322" but thread1 will try to write the
same value. It will fail because there is already a row with that
value in the ddbb.
How can I solve this problem? Can I use the F() function for this?
Should I use SQL "SELECT ... FOR UPDATE" ?
Thank you very much,
Adrian Ribao
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