python - How to order django-mptt tree by DateTimeField? -

this model using:

class comment(mpttmodel):     comment = models.charfield(max_length=1023)     resource = models.foreignkey('resource')     created_at = models.datetimefield(auto_now_add=true)     parent = treeforeignkey('self', null=true, blank=true, related_name='children')     author = models.foreignkey(user)      class mpttmeta:         order_insertion_by = ['created_at'] 

however, when try add comment admin site get:

valueerror @ /admin/app/comment/add/ cannot use none query value 

am doing wrong model? feel django-mptt trying datetimefield while still "none", before has been set @ db level.

no, you're not doing wrong. bug in django-mptt.

basically datetime fields auto_add_now=true don't value until after django-mptt tries figure out insert model in tree.

i've created issue on django-mptt fix this: https://github.com/django-mptt/django-mptt/issues/175

in meantime, can work around proactively setting value yourself. rid of auto_now_add=true, , set value in overridden save() method on model::

from datetime import datetime  class comment(mpttmodel):     comment = models.charfield(max_length=1023)     resource = models.foreignkey('resource')     created_at = models.datetimefield()     parent = treeforeignkey('self', null=true, blank=true, related_name='children')     author = models.foreignkey(user)      class mpttmeta:         order_insertion_by = ['created_at']      def save(self, *args, **kwargs):         if not self.created_at:             self.created_at = datetime.now()         super(comment, self).save(*args, **kwargs)