Really hoping someone can help---didn't get any responses on stackoverflow,
and django docs don't seem to give an exact answer.
I have a bunch of views where I check if certain conditions are true before
executing. I'm concerned about one user using a view that changes the
data after the condition has been checked in another view. Do I need to
use select_for_update in the code below?
I'm using the setting ATOMIC_REQUESTS = True, to wrap each request in a
transaction. My site is relatively low-traffic so I'm not concerned about
performance yet. Using postgres as well.
*def feed_dog(self, dog_id):*
dog = get_objects_or_404(Dog, pk=dog_id)
if not dog.removed and not dog.fed_today: *# line 3*
*# do additional checks*
dog.fed_today = True *# line 5*
dog.save()
# also modify other related objects
treat = Treats.objects.last()
treat.dogs_being_fed_with_this_treat.add(dog)
treat.save()
*def remove_dog_from_feed_list(self, dog_id):*
dog = get_objects_or_404(Dog, pk=dog_id)
if not dog.fed_today *# line 12*
dog.removed = True
dog.save()
treat.dogs_being_fed_with_this_treat.remove(dog)
treat.save()
So my concern is that someone calls the feed_dog view, and then after line
3 is checked but before line 5 is started, another user calls the
remove_dog function. Since dog.fed_today is False still when user 2 is
calling remove_dog, line 12 is True, and remove_dog view goes ahead and
does dog.removed = True. At the same time the feed_dog view continues and
sets dog.fed_today=True, adds the dog to the treat array, etc. So then I
have an inconsistent state of dog.removed = True, and dog.fed_today = True.
Basically I need it so that certain attributes on the Dog model can't both
be True at the same time, like dog.fed_today =True and dog.removed=True. I
also have this related model Treat, where I don't want a dog object to have
removed = True and also be in the treat.dogs_being_fed array.
Q:Is this a valid concern? Since transactions provide isolation, doesn't it
mean that 1 of these views accesses the data either all before or all after
the other one (they can't see the data in the half-done state)?
Q:What's the purpose of select_for_update, and would it help here? If
transactions are already supposed to provide isolation, what's the purpose
of select_for_update?
Q:Would it help to increase the isolation property of Django/Postgres from
Read Committed?
Q:Would it help to have constraints on the Dog model itself (like having a
def clean method that says a Dog can't be both removed=True and fed_today =
True?). I'm mostly checking things in the views. Would this get reflected
in a transaction? Would I also be able to create a constraint that reflects
attributes in multiple models, like can't have dog.removed=True and have
that dog be in the Treats.dogs_being_fed array?
Appreciate any help!
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