Comment 1 for bug 615766

From my previous comment:
> 1) south needs introspection rules (see http://south.aeracode.org/docs/customfields.html#extending-
> introspection), the following appear to work (with south 0.7.1 and trunk version of sortedm2m)

Actually, it doesn't work properly: the migration generated by south contains the following:
---------------------
        # Adding M2M table for field books on 'Shelf'
        db.create_table('myapp_shelf_books', (
            ('id', models.AutoField(verbose_name='ID', primary_key=True, auto_created=True)),
            ('shelf', models.ForeignKey(orm['myapp.shelf'], null=False)),
            ('book', models.ForeignKey(orm['myapp.book'], null=False))
        ))
---------------------
Which means that the sort_value column is not recognized by south, which will cause problems later on, since the column won't be created in the database