On Tue, 2008-08-26 at 08:05 -0700, [EMAIL PROTECTED] wrote:
> Hi guys,
>
> I'm trying to optimize one of my views that is doing a rather (naive?)
> query like this:
>
> ...
> post_stream = Post.objects.filter(poster=some_user)
> for other_user in other_user_queryset:
> post_stream = post_stream | Post.objects.filter(poster=other_user)
> ...
[...]
> The ORM generates a SQL query for each of the additional piped OR
> queries (this becomes a large number very quickly!).
Which version of Django are you using? Because this isn't true for the
current subversion code. How are you determining that a query is run
esach time?
Querysets are only evaluated when you first access their results and
"or"-ing together two querysets doesn't access the results, so it
constructs a single SQL query. The above loop should execute two
queries: one for the other_user_queryset (since you're accessing the
results there) and then one when you access the results of post_stream.
A more concise, and probably clearer, way to write the above loop is to
use Q() objects:
post_filter = Q(poster=some_user)
for user in other_queryset:
post_filter = post_filter | Q(poster=user)
post_stream = Post.objects.filter(post_filter)
That's a little bit more lightweight at the Python level, since it's not
constructing and copying a QuerySet object each time around. From a
style point of view it's also focusing on constructing the filter, which
is the significant thing the loop does, rather than the queryset, which
is just a way to get at the results which are filtered.
Regards,
Malcolm
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