Re: Preventing race conditions when submitting forms

On 2014-11-25 07:57, Paul Johnston wrote:
> Consider an e-commerce site, where Alice and Bob are both editing
> the product listings. Alice is improving descriptions, while Bob is
> updating prices. They start editing the Acme Wonder Widget at the
> same time. Bob finishes first and saves the product with the new
> price. Alice takes a bit longer to update the description, and when
> she finishes, she saves the product with her new description.
> Unfortunately, she also overwrites the price with the old price,
> which was not intended.

The common solution in this case your model would have something like
a "last_modified_timestamp" or "last_modified_counter". This would
be sent with the form and then resubmitted back. If the
timestamp/counter is the same, you know it's safe to update the
timestamp or increment the counter (done within the transaction to
ensure atomicity). If it's *not* the same timestamp/counter as it was
when the form was created, you know that it's been modified in the
interim and you can present a conflict-resolution form.

I don't think Django has anything like this out of the box, but it's
fairly straightforward to implement. I've done it enough times that
I should just create some model/form mixin to handle it for me.

-tkc



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