On 6/09/2010 4:19pm, kmpm wrote:
I have a project running in a manufacturing industry that is actually
built upon django.
In this I need to generate a unique serial, batch or lot number
(depending on what you would like to call it) that is a running number
from 0 to whathever for each and every day.
So when every day ticks over at midnight, the serial number starts at
zero again? Does production continue across the time 00:00? Why don't
you use a continuously increment number?
I don't know what the "right" thing to do is. I'm sure there will be a
classic solution. I think I would be writing a singleton function
nextnum() which returns the next number as required and then
lot_no = models.IntegerField(default=nextnum)
I'm not sure what would happen threadwise but a singleton would be a
natural place to deal with thread-locking and restarting the sequence
when the clock ticks over.
lot_no then gets the value prior to the save()
Mike
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.
The important part of the model looks like this...
class ProducedEntity(models.Model):
....
production_date = models.DateField(auto_now_add=True)
lot_no = models.PositiveIntegerField(default=0)
class Meta:
unique_together = ('production_date', 'lot_no')
Of course I could just save it and see if the .save() call works
without generating a IntegrityError but that's not elegant.
Is there a way of generating that number per day in a way that it's
done when I save the model?
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