Orderable ArrayAgg and StringAgg

[Floris den Hengst]

The Postgres-specific ArrayAgg and StringAgg aggregations were added in ​Django 1.9.

The ​documentation of Postgres 9.0 first mentions the possibility of ordering the results within aggregations such as ARRAY_AGG and STRING_AGG.

This could be useful in some cases.
For example: it could make sense to perform a StringAgg in lexicographical order in some cases.

The basic format of ordering within aggregations in SQL is quite simple:

SELECT ARRAY_AGG(some_field ORDER BY some_field ASC) FROM table;
SELECT ARRAY_AGG(some_field ORDER BY some_field DESC) FROM table;
SELECT ARRAY_AGG(some_field ORDER BY other_field ASC) FROM table;

It would be nice if the above would be supported as follows:

Model.objects.aggregate(ArrayAgg(some_field, order_by='some_field'))
Model.objects.aggregate(ArrayAgg(some_field, order_by='-some_field'))
Model.objects.aggregate(ArrayAgg(some_field, order_by='other_field'))

where order_by is an optional parameter. If it not specified, behavior can remain unchanged from the current implementation.

As noted by Josh Smeaton in the ​mailinglist discussion for this feature, any ordering added within may need to be contributed to GROUP BY. This might require some investigation.