On Fri, Mar 4, 2011 at 8:40 PM, Sidney Cadot <sid...@jigsaw.nl> wrote:
> Hi Tom,
>
>> Unless I'm being dense, you cannot represent this as a single SQL
>> query, so logically you cannot represent this as a QuerySet.
>
> It is possible by using a nested query, e.g.
>
> SELECT * FROM (SELECT * FROM some_table ORDER BY timestamp DESC LIMIT 10)
> ORDER BY timestamp ASC;
MySQL at least will barf on an limited subquery:
This version of MySQL doesn't yet support 'LIMIT & IN/ALL/ANY/SOME subquery'
I'm not sure what versions do, if any.
>
> But I suppose QuerySets are designed to map onto a single, non-nested query
> (I don't know the innards of Django I'm afraid) so such a construct would be
> out of reach.
> An alternative would be to augment querysets such that it would be possible
> to iterate over them in reverse order.
>
QuerySets aren't designed that way at all, most filter arguments that
take a list will also take a queryset, and the new composed queryset
will have a sub-query.
Eg:
UsageLogEntry.objects.filter(
id__in=UsageLogEntry.objects.all().order_by('-time')[:10]
).order_by('time')
would be perfectly valid as far as Django is concerned, but unstable
and non-portable from a DB perspective (MySQL won't allow that).
Reverse iterators over a queryset would be great, but would require
either caching on the queryset result (in which case, you're back to
"It's trivial in python") or reverse iterator support for the database
API, which again, don't exist.
Cheers
Tom
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