Add support for to_field_name to SortedMultipleChoiceField

[conradev]

The current implementation depends on in_bulk, which requires value to be the pk, when it can actually be any unique field thanks to the to_field_name attribute.

The super implementation handles filtering the queryset, so clean only needs to order the objects. This implementation is partially inspired by how the ModelMultipleChoiceField implements it.

[gregmuellegger]

Cool that makes sense. Thanks for taking the effort to contribute this.

I see two things missing here before I can merge this:

• The tests are failing for Python 2.6, could you change the dict comprehension to use the old style dict((key, value) ...) one?
• There are unittests missing for this feature. It would be nice to have some tests here so that can be sure enough that we don't break the support for to_field_name in the future.

[conradev]

I went ahead and made those two changes, let me know if I need to make any more! 👍

[gregmuellegger]

👍 great! Thanks for the contribution, it's merged.

[gregmuellegger]

I just released 1.3.0, containing your changes: https://pypi.python.org/pypi/django-sortedm2m/1.3.0

[conradev]

Awesome, thanks! 🎉

[vqw]

@conradev @gregmuellegger
I have the same issue as @czpython in DjangoCMS which drives me crazy. Reverting to previous version.