allow querying of existance of backwards relationship without resorting to custom sql

[mattimustang@…]

Both of these threads cover the same issue:

​http://groups.google.com/group/django-users/browse_frm/thread/98b7aedf8f9d806c

​http://groups.google.com/group/django-users/browse_frm/thread/b129d64b1ed77e28

To summarize, I can get all Reporters who have associated Articles by doing:

        Reporter.objects.filter(article__isnull = False) 

But, I can't get Reporters who have *no* associated Article by doing:

        Reporter.objects.filter(article__isnull = True) 

as it returns an empty list, since the article property does not exist for
those Reporters, and thus, it can't even be null.

Furthermore, being able to do the same in limit_choices_to would be extremely useful.

[Tyson Tate <tyson@…>]

+1 Would be useful. Sign me up for this one.

[Malcolm Tredinnick]

(In [6503]) queryset-refactor: Added a test to show we can now query for empty reverse
relationships. Refs #2400.

[Malcolm Tredinnick]

(In [7477]) Merged the queryset-refactor branch into trunk.

This is a big internal change, but mostly backwards compatible with existing
code. Also adds a couple of new features.

Fixed #245, #1050, #1656, #1801, #2076, #2091, #2150, #2253, #2306, #2400, #2430, #2482, #2496, #2676, #2737, #2874, #2902, #2939, #3037, #3141, #3288, #3440, #3592, #3739, #4088, #4260, #4289, #4306, #4358, #4464, #4510, #4858, #5012, #5020, #5261, #5295, #5321, #5324, #5325, #5555, #5707, #5796, #5817, #5987, #6018, #6074, #6088, #6154, #6177, #6180, #6203, #6658