Have a look at this thread:
http://groups.google.com/group/django-users/browse_thread/thread/deda522d2545325e/bbaa8edbeda204e3?lnk=gst&q=production+number#bbaa8edbeda204e3
The author has a similar problem i.e. he says:
"There is a high risk of concurrency so just finding the previous max
and then do a +1 before saving is not what I want. "
On Oct 21, 11:54 pm, orokusaki <flashdesign...@gmail.com> wrote:
> Correction: I meant for the third ticket to have pk: 3.
>
> On Oct 21, 3:18 pm, Yo-Yo Ma <baxterstock...@gmail.com> wrote:
>
> > Example:
>
> > Company has many Tickets
> > Tickets have a PK, as well as a "number".
> > Each Ticket's "number" should be the highest prior "number" for a
> > Ticket with the same Company
>
> > Ticket: pk: 1, number: 1, company: XYZ
> > Ticket: pk: 2, number: 1, company: Acme
> > Ticket: pk: 1, number: 2, company: XYZ
>
> > unique_together=(("number", "company"),) # number is not unique
> > itself.
>
> > How should I handle this non-sense?
>
> > Confusing, convoluted, in-depth (sort of) explanation:
>
> > I have a model that sets the value of one of it's fields in the save()
> > method. It does this by reading the value of another field, and using
> > that value to calculate the value based on how many instances there
> > are with the same value in the other field (Basically an auto field
> > that increments with regards to a foreign key). The field that save()
> > sets is a unique field. This means that if 2 people on other sides of
> > the globe hit "submit" at the same time, the number of instances with
> > "other field"s value will be the same (e.g. 500). Then save() will
> > make self.number = 501, causing an IntegrityError for which ever one
> > takes longer to finish. How can I get around this? Or, better yet: How
> > should I do this instead?
>
> > Thanks in advance.
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