> Interestingly enough, just doing a .filter(m2m_relation__foo='bar') might
change the results
Is that not because it's a filter? Would ".
annotate(x=F('m2m_relation__foo'))" change the results in any way? Sorry if
I'm not following.
On 21 Aug 2017 06:58, "Anssi Kääriäinen" <akaar...@gmail.com> wrote:
On Sunday, August 20, 2017 at 2:48:23 AM UTC+3, Josh Smeaton wrote:
> Thanks for making me elaborate, because I'm probably wrong. I'll step
> through what my thinking was though.
>
> '.annotate(local=F('related__field'))' will join to the related table. If
> `related` is a nullable foreign key, then a join would do an implicit
> filter, removing the row from the result set if it had no relation, and
> reducing the count.
>
> But Django models a nullable foreign key join as a LEFT JOIN which
> wouldn't filter the row out and wouldn't reduce the count.
>
> Multivalue relationships (reverse foreign key and m2m joins) would need
> some care though I think. I'm not sure if the same annotation above works
> on m2m joins or not.
>
Interestingly enough, just doing a .filter(m2m_relation__foo='bar') might
change the results. For m2m relations, any non-aggregate annotation is not
safe to remove.
I believe our test cases are not fully covering here, so removing the
annotations will be a bit risky. If there are cases where removal is known
to be safe, it's of course a good idea to remove anything extra from the
query.
- Anssi
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