Duplicate rows with same key in DB?

[ydjango]

I have code as

view_a(request):
  .... Populate my_a, my_b and my_c...
  try:
      tp = TP.objects.get(a__exact = my_a, b__exact = my_b)
  except TP.DoesNotExist, e:
     tp = TP()
     tp.a = my_a
     tp.b = my_b
     tp.c = my_c
     tp.save()
  else:
     tp.c = my_c
     tp.save()

This view is called by ajax calls. TP is a django model and mysql db.
I am using django 1.1.4/ python2.5/mod_wsgi2.5,

I find that in, one case, two rows with same value of  a and b has been created in DB. ( value of c was different)
What could have caused it?

Any clues or hints on how to track down this problem or what to look for will be appreciated.

[Daniel Roseman]

You probably have a race condition. You should use the built-in `get_or_create` manager method, which helps to minimize that sort of thing (it's not perfect, but it's better than coding it yourself).

--

DR.

[Kurtis Mullins]

You could also put a unique_together statement in your Model. If you do this, you'll need to update your database as well to enforce the constraint at the database level.

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[ydjango]

Thank you, I also believe it is Race condition. I do not want to use get_or_create as I like to have control over DB writes and updates. I will try Unique_together and catch the integrity error exception as Kurtis suggested or check again before doing save().

or just thinking aloud, Is there a way to acquire a lock which can serialize any other requests which have same "a" and "b" values?

[Melvyn Sopacua]

On 23-6-2012 0:02, ydjango wrote:
>
> Thank you, I also believe it is Race condition. I do not want to use
> get_or_create as I like to have control over DB writes and updates. I will
> try Unique_together and catch the integrity error exception as Kurtis
> suggested or check again before doing save().
>
> or just thinking aloud, Is there a way to acquire a lock which can
> serialize any other requests which have same "a" and "b" values?

unique_together + transaction middleware.
https://docs.djangoproject.com/en/1.4/topics/db/transactions/
--
Melvyn Sopacua

[Melvyn Sopacua]

Ok, hit send too soon. No, this doesn't give you access to other
requests as far as I can tell, but operations depending upon the insert
operation succeeding are now part of a transaction and can be handled
more gracefully inside django as well, pending the result of the
transaction.

--
Melvyn Sopacua

[Cal Leeming [Simplicity Media Ltd]]

Also - get_or_create doesn't always prevent a race condition if you have high traffic.. depending on your code+stack.

I already did a post about this a while back:

http://markmail.org/message/j3qr2y26i5uk7c3z 

--
Melvyn Sopacua

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