Cannot use Aggregation function in Model.Meta.ordering

[wilhelmhb]

Modifying the example from ​https://docs.djangoproject.com/en/2.0/topics/db/queries/# to add ordering:

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=200)

class Entry(models.Model):
    headline = models.CharField(max_length=255)
    authors = models.ManyToManyField(Author)

    def __str__(self):
        return self.headline

    class Meta:
        ordering = [
            models.Min('authors__id').asc(),
            models.Max('authors__id').asc()
        ]

The query to get entries will fail with a ProgrammingError: the SQL query doesn't contain the GROUP BY clause.
As aggregation functions are QueryExpression objects, and the ordering field handles QueryExpressions since Django 2.0, it should work.

[Tim Graham]

I'm not sure how this should be solved. The problem is that adding that order_by() clause to a model without Meta.ordering doesn't work:

>>> Entry.objects.order_by(models.Min('authors__id').asc(), models.Max('authors__id').asc()))
...
django.core.exceptions.FieldError: Using an aggregate in order_by() without also including it in annotate() is not allowed: OrderBy(Min(F(authors__id)), descending=False)

[Can Sarıgöl]

I tried to fix this. Could you review? ​PR

[Tim Graham]

Removing the deprecation from #14357 isn't appropriate.

[Tim Graham]

From Simon on the pull request:

I think this feature would be hard to get right for the rare case where it's useful. It also go against our stance on trying to disconnect Meta.ordering from aggregation (#14357) as this PR demonstrates. I'd be in favour of won't fixing the ticket as well for these reasons. It feels incoherent with the ongoing deprecation motivated by observed user confusion over the years from the interactions between these two features.