Query Expression in ordering of a related object fails

[wilhelmhb]

Since 2.0, according to the doc (​https://docs.djangoproject.com/en/2.0/ref/models/options/#ordering), we can use QueryExpression objects in the Model.Meta.ordering field.
Using:

from django.db import models

class Musician(models.Model):
    first_name = models.CharField(max_length=50)
    last_name = models.CharField(max_length=50)
    instrument = models.CharField(max_length=100, null=True, blank=True)

    class Meta:
        ordering = [models.F('instrument').asc(nulls_last=True)]

class Album(models.Model):
    artist = models.ForeignKey(Musician, on_delete=models.CASCADE)
    name = models.CharField(max_length=100)
    release_date = models.DateField()
    num_stars = models.IntegerField()

    class Meta:
        ordering = ['artist']

>>> Album.objects.all()
...
TypeError: 'OrderBy' does not support indexing

When reaching ​https://github.com/django/django/blob/master/django/db/models/sql/compiler.py#L669, the compiler tries to use the related model, but at line 679, item can be an OrderBy object. Thus the failure.

[Alexandre Laplante]

Started working on a fix.

[Alexandre Laplante]

​PR Looking for feedback on this patch.