Migrating with South on a field that uses auto_now_add=True

16 December 2009   5 comments   Django

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I have a Django model that looks something like this:

class MyModel(models.Model):
   modify_date = models.DateTimeField(auto_now=True)

Retroactively now I wanted to add a field called add_date which uses the auto_now_add=True trick. The migration used in this project is South which is great but doesn't work very well with the auto_now_add=True because the field doesn't have a straight forward default. So, first I changed the field to this:

class MyModel(models.Model):
   modify_date = models.DateTimeField(auto_now=True)
   add_date = models.DateTimeField(auto_now_add=True, null=True)

Notice the null=True which is important. Then I used startmigration to generate the code for the forward and backward to which I added a this stuff:

class Migration:

   def forwards(self, orm):

       db.add_column('myapp_mymodel', 'add_date', orm['myapp.mymodel:add_date'])
       for each in MyModel.objects.all():
           # since MyModel is referenced elsewhere I can work out the oldest date
           oldest_date = get_oldest_related_date(each, 
           each.add_date = oldest_date

That way all old records will have the date (not entirely accurate but good enough) and all new records will automatically get a date. Is there a better way? I bet, but I don't know how to do it.


I think the better way is to avoid using auto_now_add in the first place, and instead use the more explicit "default=datetime.datetime.now" (or utcnow). Then South can handle it without assistance.
Carl: Using "default=datetime.datetime.now" doesn't work with South migrations. It causes South to evaluate the expression, hard-coding today's date in the migration. Then every time you run the migration you get that date, regardless of the actual date.
Hmm, apparently South 0.6 doesn't actually handle dynamic defaults well at all; I was basing this comment off the last time I did it, with 0.5, when it worked fine. So never mind ;-)

Though apparently the next version of South will handle it well again (by ignoring defaults entirely): http://south.aeracode.org/ticket/273
We use something like the following when migration data containing fields that have auto_now or auto_now_add:

#Hack must override the pre_save to get touched to update correctly.
def no_touch_pre_save(self, model_instance, add):
return super(models.fields.DateField, self).pre_save(model_instance, add)
#End override touched.

def no_auto_add(fn):
Hack django to not update touched - used for migrations.
def _no_auto_add(*args, **kwargs):
#Force users to be explicit in this method about touched fields.
old_pre_save = models.fields.DateField.pre_save
models.fields.DateField.pre_save = no_touch_pre_save

#Call fn
ret = fn(*args, **kwargs)

#Revert to old skool pre_save in case.
models.fields.DateField.pre_save = old_pre_save
return ret
return _no_auto_add
Fabian Topfstedt
In your case I would monkeypatch the fields of MyModel while migrating:

from django.db.models.fields import DateTimeField
from my_app.models import MyModel

def forwards(self, orm):
for f in [f for f in MyModel._meta.fields if isinstance(f, DateTimeField)]:
f.auto_now_add = False
f.auto_now = False
# Your data migration code here...

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