last() 和 [] 运算符给出不同的结果

Question

I have a model RSSFeed. To get the last element in my DB, I do:

RSSFeed.objects.last()
# Output: <RSSFeed: www.sooperarticles.com>

I slice it to get the first 10 element in the query

first_ten_feeds = RSSFeed.objects.all()[:10]

Using first and the bracket operator is consistent:

first_ten_feeds.first()
# Output: <RSSFeed: pressetext News>
first_ten_feeds[0]
# Output: <RSSFeed: pressetext News>

But using last and the bracket operator is not consistent:

first_ten_feeds[9]
# Output: <RSSFeed: FinanzNachrichten.de: Nachrichten zu IT-Dienstleistungen>

first_ten_feeds.last()
# Output: <RSSFeed: www.sooperarticles.com>

Why? I expect to get the same result for last() and [] above.

RSSFeed.objects.last() and first_ten_feeds.last() seem to give the same result but that does not make sense to me.

Answer 1

The answer is in the code actually. QuerySet.last() is defined as

    for obj in (self.reverse() if self.ordered else self.order_by('-pk'))[:1]:
        return obj

and what QuerySet.reverse() basically does is to return a clone of the queryset with the direction of the order by clause reverted , so basically instead of

SELECT (...) from yourmodel ORDER BY somefield ASC LIMIT 10

the SQL query becomes:

SELECT (...) from yourmodel ORDER BY somefield DESC LIMIT 10

so first_ten_feeds.last() will actually returns the same thing as RSSFeed.objects.last() .

This behaviour doesn't really match the doc and is quite surprising, not to say totally unexpected, and I strongly suggest you fill in a bug report on django's issue tracker - either it's the expected behaviour (for the django devs at least) and then it should be clearly documented, or it's a plain bug.

Answer 2

Just crossed with this question, I have a similar case:

 elem = my_qs.last()
 elem in my_qs
>>> False
 elem
>>> <1 - 'elem 1'>    # <- wrong
 elem = list(my_qs)[-1]
 elem
>>> <2 - 'elem 2'>    # <- correct

Still don't know the reason for this.

I also tried to apply some ordering, as suggested for later versions of Django, but the result is the same.

elem = my_qs.all().order_by('pk').last()
elem
>>> <1 - 'elem 1'>