views:

832

answers:

4

I've done a bit of reading related to the concurrency issues with sqlite, but I don't see how they'd apply to Django since it's inherently single threaded. I'm not using any multiprocess modules either. I have absolutely no experience with concurrent programming either, so if someone can identify WHY the following code is causing an OperationalError: 'database is locked' I'd be grateful.

views.py

def screening(request, ovramt=None):
errors = []
if request.method == "POST":
    form = ScreeningForm(request.POST)
    if form.is_valid():
       print "Woo valid!!"
    return HttpResponse()

else: # GET            
    if ovramt is None:
        o = Ovramt.objects.select_related(depth=1).latest("date_completed")
        print "found?"
        print o.id
    else:
        try:
            o = Ovramt.objects.select_related(depth=1).get(id=ovramt)
        except:
            errors.append("OVRAMT NOT FOUND") 


    if o.residents.count() <= 0:
        o.add_active_residents()
    residents = list(o.residents)

models.py

def add_active_residents(self):
    ssa_res = SSA_Resident.objects.select_related(depth=1).filter(ssa=self.ssa, active=True)
    for r in ssa_res:
        self.residents.add(r.resident) # Fails Here
    self.save()

The add_active_residents method works fine, until it is called from the views module. Is there an open connection to the database open in the view which prevents writing from the model? Does someone have an explanation why this code will error?

+3  A: 

In the following method function

def add_active_residents(self):
    ssa_res = SSA_Resident.objects.select_related(depth=1).filter(ssa=self.ssa, active=True)
    for r in ssa_res:
        self.residents.add(r.resident) # Fails Here
    self.save()

Why is there a select_related? You only really need the FK's of ssa_res items. Why do additional queries for related items?

S.Lott
SSA_Resident is the Many2Many table between SSA and Resident, which contains resident and ssa objects. I'm assuming that these objects need to be returned in order to add them to the many2many relationship of Ovramt_Resident.
Josh Smeaton
"I'm assuming..." that's my point. Try taking it out. See what happens.
S.Lott
I'm with S. Lott. Take out the select_related calls, and see if it still locks.
Matthew Christensen
The issue stopped happening after I manually added and saved the result on the command line, and wrote some new code. I'm going to assume this is correct however, but I'll do some more testing later.
Josh Smeaton
+1  A: 

Sounds like you are actually running a multithreaded application, despite what you say. I am a bit clueless about Django, but I would assume that even though it might be single-threaded, whatever debugging server, or production server you run your application in won't be "inherently single threaded".

Ali A
A: 

Are you using Python 2.6?

If so, this is (apparently) a known issue that can be mitigated by adding:

DATABASE_OPTIONS = {'timeout': 30}

to your settings.py

See http://code.djangoproject.com/ticket/9409

Matthew Christensen
I tried that solution and it didn't work. I was hoping that someone would be able to identify, in my code, what might be causing the issue
Josh Smeaton
+2  A: 

My understanding is that only write operations will result in a db-locked condition. http://www.sqlite.org/lockingv3.html

It's hard to say what the problem is without knowing how django is handling sqlite internally.

Speaking from using sqlite with standard cgi, I've noticed that in some cases it can take a 'long' time to release the lock. You may want to increase the timeout value mentioned by Matthew Christensen.

monkut