Hi Tom,
I really like the look of Count('emails').filter(unread=False) in the
annotate call.
Ian
On 12/04/17 21:35, Tom Forbes wrote:
> Hello,
> At the Djangocon sprints I wanted to add support for a postgres specific
> syntax for filtering aggregations, which is quite simple: MAX(something)
> FILTER (WHERE x=1).
>
> During this the sprints I was told that it would be good to support this
> for all databases, and it can be done using the CASE syntax: MAX(CASE
> WHEN x=1 THEN something END).
>
> Doing this with the ORM is quite verbose, it would be really nice to be
> able to have a shorthand syntax for filtering aggregates that can use
> database-specific syntax where available (the postgres syntax is faster
> than the CASE syntax). I've made a proof of concept merge request that
> implements this here:
>
> https://github.com/django/django/pull/8352
>
> I'd really like some feedback and to maybe have a discussion about what
> the API could look like. Currently I went for adding `.filter()` and
> `.exclude()` methods to the base Aggregate class. I like this approach
> as it's close to the familiar queryset syntax but I understand there are
> subtleties with combining them (I just AND them together currently).
> It's also better to be consistent - if we can't support all of the
> queryset filter() syntax then we shouldn't confuse people by having a
> filter method that acts differently.
>
> An example of the API is this:
>
> Mailboxes.objects.annotate(
>read_emails=Count('emails').filter(unread=False),
>unread_emails=Count('emails').filter(unread=True),
>recent_emails=Count('emails').filter(received_date__lt=one_week_ago)
> )
>
>
> I'd really like some feedback on what the API should look like. Is
> filter a good idea? Any feedback on the current merge request
> implementation is appreciated as well, I'm fairly new to the Django
> expression internals. I think I'm going the right way with having a
> separate FilteredAggregate expression but I'm not sure.
>
> Many thanks,
> Tom
>
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