Anthony, thank you.
I didn't know that something like session[myvar] would work. I
thought it was only dot notation.
Thanks for the pointer on form.vars.
I'll look for more info on form.process().
I would be more worried about a record being left in a locked
condition than I would be about a race condition. What happens if the
client goes away and never updates the record?
I know the code was sloppy. I hope nobody uses it for an example.
On Oct 27, 2:18 pm, Anthony <abasta...@gmail.com> wrote:
> How about:
>
> yield_key = 'old_job_yield_%s' % request.args(0)
> session[yield_key] = old_job_yield
>
> That way, the session key is unique to the particular job being edited, and
> editing another job in a different tab won't overwrite the old_job_yield
> associated with this job. That won't protect against the user potentially
> editing the _same_ job in two different tabs (though to protect against
> that, you could use the SQLFORM detect_record_change argument), but should
> be OK for separate jobs. There is still a very small opportunity for a race
> condition in between reading the current value of quantity_on_hand and
> updating that value, but that would require two edit forms being submitted
> nearly simultaneously (actually, I think trunk now includes a feature
> allowing you to select a record for update, so it will remain locked until
> updated).
>
> A few other issues:
>
> - Should be request.args(0), not request.vars(0) (request.vars is like a
> dictionary -- access items with keys, not subscripts).
> - Should be form.accepts(request, session), not (session, request) --
> though, now, you should use form.process().accepted
> - After form acceptance, it's probably better to refer to form.vars
> (i.e., the validated values) rather than request.post_vars.
>
> Anthony
>
>
>
>
>
>
>
> On Thursday, October 27, 2011 9:54:21 AM UTC-4, Cliff wrote:
>
> > What is the Web2py way of making session variables unique to one
> > browser tab? This example illustrates the problem.
>
> > Model:
> > db.define_table('products', Field('quantity_on_hand', 'integer'))
> > db.define_table('jobs', Field('job_yield', 'integer'),
> > Field('product_id', db.products))
>
> > When a user updates the job_yield field, the jobs controller also
> > needs to update quantity on hand in the products table. Usually one
> > would use a session variable something like this.
>
> > def edit():
> > # fetch current job yield
> > old_job_yield = db.jobs[request.vars(0)].job_yield
> > # save it in a session
> > session.old_job_yield= old_job_yield
> > ...
> > if form.accepts(session,request):
> > if request.post_vars.job_yield != session.old_job_yield:
> > yield_delta = request.post_vars.job_yield -
> > session.old_job_yield
> > # assume we magically fetch related product id, then
> > old_quantity_on_hand =
> > db.products[product_id].quantity_on_hand
> > new_quantity_on_hand = old_quantity_on_hand +
> > yield_delta
>
> > db(db.products.id==product_id).update(quantity_on_hand=new_quantity_on_hand
> > )
>
> > The problem arises if the user edits a different job in a different
> > tab. Because there is only one instance of session.old_job_yield, the
> > edit action in the other tab will overwrite it.
>
> > What is the Web2py way of handling this problem?
>
> > Thanks,
> > Cliff Kachinske
