Admin renders a sort control for every "__str__" column

[blighj]

Since ticket:10743#comment:26, any __str__ column in an admin list view is rendered as a sortable column, regardless of whether it has been linked to a column and marked as sortable.
This simple model would reproduce it.

# models.py
class SortingTest(models.Model):
    name = models.CharField(max_length=30)

    def __str__(self):
        return self.name


# admin.py
admin.site.register(SortingTest)

list_display defaults to ("__str__",) but no sorting has been configured for the model's __str__, so the column should not be sortable.

result_headers() in django/contrib/admin/templatetags/admin_list.py
decides sortability with a substring test:

if not admin_order_field and LOOKUP_SEP not in field_name:
    is_field_sortable = False

LOOKUP_SEP is "__" an so __str__ flags as sortable

ChangeList.get_ordering_field() resolves the same name by attribute lookup
rather than by checking for double underscores, correctly finds no admin_order_field, and
get_ordering() drops it:

order_field = self.get_ordering_field(field_name)
if not order_field:
    continue

Potentially result_headers could use get_ordering_field or some other technique.

This showed up reviewing ticket:27752#comment:16, which has the bigger issue that sorting on __str__ doesn't work anyway. I don't judge this as a huge issue as once you need sorting you are most likely using specific list_dispaly values.