Django South datamigration pre_save() uses model's __unicode__()

Question

My Blog model has AutoSlugField which uses Blog.__unicode__() method.

After data migration all Blog instances have slug set to blog-object-<number> instead of <year>-<month>-<day>. Seems like definition Blog.__unicode__() is ignored.

How could I correctly migrate Blog model?

modelfields.py:

class AutoSlugField(models.CharField):
    def pre_save(self, blog, *args, **kwargs):
        return slugify(unicode(blog))

models.py:

class Blog(models.Model):
    title = models.CharField(max_length=255)
    created = models.DateTimeField(auto_now_add=True)
    slug = AutoSlugField(max_length=50)

    def __unicode__(self):
        return self.created.strftime('%Y-%m-%d')

Migration:

from south.v2 import DataMigration

class Migration(DataMigration):
    def forwards(self, orm):
        for blog in orm.Blog.objects.all():
            blog.title = blog.title.replace('django', 'Django')
            blog.save() 

Answer 1

I have updated to South 0.7.6 and used solution from the South documentation. Simply added to_python() and get_prep_value() methods to leave slug field as is.

class AutoSlugField(models.CharField):
    def pre_save(self, blog, *args, **kwargs):
        return slugify(unicode(blog))

    def to_python(self, value):
        return value

    def get_prep_value(self, value):
        return value

Answer 2

South does nothing but just add column to your table and django has no role to play in it. So when you run migration, django model save method is not called hence no pre-save method gets called. South works on database only, i.e., you can provide attributes such as default value, nullable etc. which can be set at db level. To add slug to existing records in db, create util function which would slugify your field or write a data migration.