QuerySet.last() and [] operator give different results

[Martin Ennemoser]

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.

I already openend a SO question with the same content: ​https://stackoverflow.com/questions/54477472/last-and-operator-give-different-results
There I was suggested to open a bug.

[Tim Graham]

As far as I can tell from the code you gave, the slice is operating on an unordered queryset. There's no guarantee as to what ordering the database will return. last() gives ordering by primary key if the queryset isn't ordered.