filter keyword undocumented in PostgreSQL aggregate functions

[Matthew Pava]

The signature for all of the aggregate functions for PostgreSQL includes a filter keyword argument, but I can't figure out what it is used for based on the documentation alone: ​https://docs.djangoproject.com/en/2.0/ref/contrib/postgres/aggregates/.

For what it's worth, I'm trying to filter out all of the empty values in an ArrayAgg expression, which seems to be a fairly common thing that one would want to do. Perhaps we could add an example to the documentation showing how that can be accomplished. I'm not sure right now if the filter keyword argument is the way to go.

[Tim Graham]

The relevant commit is b78d100fa62cd4fbbc70f2bae77c192cb36c1ccd. I don't think it makes much sense to repeat the documentation for filter in every aggregate subclass. Ideas?

[Matthew Pava]

I agree. I just found the documentation here:
​https://docs.djangoproject.com/en/2.0/topics/db/aggregation/#filtering-on-annotations
Honestly, though, even that little note seems a little buried in everything.

Maybe we can add a link to the generic aggregation documentation on the PostgreSQL aggregation documentation?
And, I think I found a bug. I keep getting this error when using a filter Q object with ArrayAgg:
AttributeError: 'WhereNode' object has no attribute '_output_field_or_none'

[Scott Halgrim]

proposed fix here ​https://github.com/shalgrim/django/tree/ticket_29317

[Tom Forbes]

Hey shalgrim, can you make a pull request to the Django project with this? If you visit ​https://github.com/django/django you should see a button to do this

[Scott Halgrim]

Oh I'm sorry guys. Two things. I had meant to link to my PR before: ​https://github.com/django/django/pull/10530

And I just saw Tom's comment right now.

Again, sorry for the confusion. Thanks, Vishavjit

[Tim Graham <timograham@…>]

In 7e9d5a0a:

[2.1.x] Fixed #29317 -- Doc'd filter argument in contrib.postgres aggregates.

Backport of 926fa7116fd633b69277c3ad9b3370ca45163231 from master.

[Tim Graham <timograham@…>]

In 926fa71:

Fixed #29317 -- Doc'd filter argument in contrib.postgres aggregates.