added reverse order for admin_order_field

[fetzig]

after adding this you can setup something like this:

def age_in_years(self):
    return # age calculated via DateField self.date_of_birth
age_in_years.admin_order_field = '-date_of_birth'

the '-' at the beginning of '-date_of_birth' indicates the reverse order in the changelist.

already made a pull request here:
​https://github.com/django/django/pull/2201

and patch is here:
​https://github.com/fetzig/django/commit/2924431df1060d5ec1e0d2e6778b3c58cb85de05.patch

missing:

• patch for docs
• test(s)

won't be able to make the docs update and tests within the next week. so everybody is very welcome if you like to do that :)

[fabulousmuscles]

I added a test and documentation for this new feature.
My pull request is here: ​https://github.com/django/django/pull/2275
All tests pass under SQLite. This was my first time contributing to the Django project and I think I did everything correctly but if there's anything 'off' please let me know. Thanks.

[Tim Graham <timograham@…>]

In f683cb90bea2afbe0ef4c011acd4ab590c37410d:

Fixed #21924 -- Added the ability to specify a reverse order for admin_order_field.

Thanks Klemens Mantzos for the report and initial patch.