[Django] #34145: It is currently not possible to group by annotated fields

[Django]

#34145: It is currently not possible to group by annotated fields
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Reporter: Karolis Ryselis | Owner: nobody
Type: Bug | Status: new
Component: Uncategorized | Version: 4.1
Severity: Normal | Keywords:
Triage Stage: Unreviewed | Has patch: 0
Needs documentation: 0 | Needs tests: 0
Patch needs improvement: 0 | Easy pickings: 0
UI/UX: 0 |
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Consider the following models:
{{{#!python
class PackingTag(models.Model):
title = models.CharField(max_length=64)

class Packing(models.Model):
tags = models.ManyToManyField(to='PackingTag')

class PackingItem(models.Model):
packing = models.ForeignKey(to='Packing', on_delete=models.CASCADE)
amount = models.DecimalField(max_digits=12, decimal_places=4)
}}}

Consider a queryset like this:

{{{#!python
PackingItem.objects.annotate(tags=Min('packing__tags__title')).values('tags').annotate(total_amount=Sum('amount'))
}}}

Currently on both 4.1 and 3.2 it yields this query using SQLite backend
(MySQL backend on 3.2 outputs a very similar query as well):

{{{
SELECT MIN("packings_packingtag"."title") AS "tags",
CAST(SUM("packings_packingitem"."amount") AS NUMERIC) AS "total_amount"
FROM "packings_packingitem"
INNER JOIN "packings_packing" ON
("packings_packingitem"."packing_id" = "packings_packing"."id")
LEFT OUTER JOIN "packings_packing_tags" ON
("packings_packing"."id" = "packings_packing_tags"."packing_id")
LEFT OUTER JOIN "packings_packingtag" ON
("packings_packing_tags"."packingtag_id" = "packings_packingtag"."id")
}}}

I would expect this queryset to either group by tags (this example may not
make much sense, my use case involves GroupConcat aggregate from
django_mysql package, however, this is the same for all aggregates)
because passing a regular field expression constructs a group by in the
SQL, or crash with an error telling that grouping by annotations is not
supported.

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Ticket URL: <https://code.djangoproject.com/ticket/34145>
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