On 3 July 2012 06:27, Maxime Haineault <[email protected]> wrote:
> One of the common pitfall I come across way to often with novices is
> something like this:
>
> def getFirstUser():
> return User.objects.all()[0]
>
> It looks innocuous and often wont raise any exceptions in dev because you
> develop and test with data,
> so they slip easily in production.
>
> def getFirstUser():
> return User.objects.all().first()
>
> If the QuerySet is empty it should just return None instead of raising an
> exception.
>
> Other methods like .last() could probably also be useful, but getting the
> first element of a QuerySet
> is the pitfall I see the most often.
>
> Surprise Poney Quizz: which is the fastest way to get the first object of a
> QuerySet ?
>
> A) Using count:
>
> qs = User.objects.all()
> if qs.count() > 0:
> return qs[0]
> else:
> return None
>
> B) Convert to list:
>
> r = list(qs[:1])
> if r:
> return r[0]
> return None
>
>
> C) Using len:
>
> qs = User.objects.all()
> if len(qs) > 0:
> return qs[0]
> else:
> return None
>
> D) Try/except:
>
> qs = User.objects.all()
> try:
> return qs[0]
> except IndexError:
> return None
>
E) Use __bool__ which fetches only one batch (depends on DB):
qs = User.objects.all()
user = None if not qs else qs[0]
F) Use the iterator protocol
user = next(iter(User.objects.all()), None) # you can add LIMIT or
not, see Alex's comment
--
Łukasz Rekucki
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