Multiple ajax request (Singleton Pattern) - django

Question

I am using django restless for an ajax POST request, which took almost 10 to 20 seconds.
Here is my code.
class testEndPoint(Endpoint):
def post(self, request):
testForm = TestEmailForm(request.data)
if testForm.is_valid():
sometable = EmailTable.object.get(**condition)
if sometable.is_email_sent == false:
#Send Email
#Took around 15 seconds
sometable.is_email_sent = true
sometable.save()
else:
result = testForm.errors
return serialize(result)
i am calling it via $.ajax, but the problem is if two request hit this url with milliseconds time difference, both request passed through if sometable.is_email_sent = false: condition.
How can i prevent multiple submission. Right now i have moved sometable.is_email_sent = true;sometable.save(); before email send part, but i need more generic solution as there are dozen more places where this is happening. I am on django 1.5
django restless

Answer 1

You should disable the originating input element before you start your ajax call (that will prevent the majority of these issues).
The remaining problems can be solved by using select_for_update
class testEndPoint(Endpoint):
#transaction.commit_manually
def post(self, request):
testForm = TestEmailForm(request.data)
if testForm.is_valid():
condition['is_email_sent'] = False
try:
rows = EmailTable.objects.select_for_update().filter(**condition)
for row in rows:
row.is_email_sent = True
row.save()
#Send Email
except:
transaction.rollback()
raise
else:
transaction.commit()
else:
result = testForm.errors
return serialize(result)
select_for_update will lock the rows until the end of the transaction (i.e. it needs to be inside a transaction). By adding is_email_sent=False to the condition, we can remove the if. I've moved the changing of is_email_sent above the "Send Email", but it is not strictly necessary -- in any case it will be undone by the transaction rolling back if there is an exception.