Remove one-arg limitation from django.db.models.aggregate

[Anssi Kääriäinen]

The resolve_expression() method of django.db.models.aggregates.Aggregate asserts that there is just single source expression. I don't see the reason for this limitation. There are aggregates (like SQLite's group_concat) that naturally take in multiple arguments. It seems group_concat works just fine with multiple source expressions if the assertion is removed.

[Anssi Kääriäinen]

This should be one-line change to django/db/models/aggregates, remove the line that does the assert for len(self.source_expressions) and things should work.

[Greg Chapple]

PR here: ​https://github.com/django/django/pull/4767

[Andriy Sokolovskiy]

I think it because ​https://github.com/django/django/blob/3e9b5bfd9c3e16c968278f7d050f52739690eb29/django/db/models/aggregates.py#L21 is using only first expression, so it will be confusing to allow passing more

[Simon Charette]

Does len(self.source_expressions) == (c.source_expressions) in all cases? If that's the case maybe we should loop over all c.source_expressions to test for .contains_aggregate and not summarize.

[Josh Smeaton]

Yes, a loop will do. We'll also need to fail if an aggregate is provided multiple source_expressions but no alias. See ​https://github.com/django/django/blob/3e9b5bfd9c3e16c968278f7d050f52739690eb29/django/db/models/aggregates.py#L22 and ​https://github.com/django/django/blob/3e9b5bfd9c3e16c968278f7d050f52739690eb29/django/db/models/aggregates.py#L34

[Greg Chapple]

I've updated the PR to loop over source_expressions. Wondering how I can test the new behavour - can anyone give some pointers on how I might make source_expressions have more than one element?

[Greg Chapple]

Just updated the PR with a test

[Josh Smeaton]

I've extended @ghcp's PR with code that handles default_alias and a recursive contains_aggregate. ​https://github.com/django/django/pull/4900

[Tim Graham <timograham@…>]

In 4a66a692:

Fixed #24887 -- Removed one-arg limit from models.aggregate

[Simon Charette <charette.s@…>]

In 160969d:

Refs #24887 -- Stopped mutating a test expression during as_sql().

Also defined an explicit output_field as it was mixing an expression with an
IntegerField() with one with a DecimalField().