Django - Populate DateTime field from other DateTimeField set with auto_now -

i'm trying populate last_version_date modified field set auto_now. in save method, if new version created want save modified date record.

simplified example :

class poll(models.model):     question = models.charfield(max_length=200)     modified = models.datetimefield(auto_now=true)     version = models.integerfield(blank=false, default=1)     last_version_date = models.datetimefield(auto_now_add=true,blank=false)      def save(self, keep_history=false, *args, **kwargs):             if keep_history:             self.last_version_date = self.modified             self.version = self.version + 1          return super(poll, self).save(*args, **kwargs) 

usage

>>> p = poll.objects.create(question = 'test1') >>> p.save() >>> p.modified datetime.datetime(2014, 8, 18, 6, 50, 41, 820000, tzinfo=<utc>) >>> p.last_version_date datetime.datetime(2014, 8, 18, 6, 50, 38, 381000, tzinfo=<utc>) >>> p.created datetime.datetime(2014, 8, 18, 6, 50, 38, 380000, tzinfo=<utc>) >>> >>> >>> p.save(true) >>> p.modified datetime.datetime(2014, 8, 18, 6, 51, 1, 804000, tzinfo=<utc>) >>> p.last_version_date datetime.datetime(2014, 8, 18, 6, 50, 41, 820000, tzinfo=<utc>) 

notice how in second save last_version_date being updated previous value of modified.

looking @ source datetimefield's auto_now option see it's being populated in pre_save hook expect value populated prior save.

thanks.

from django documentation :

datefield.auto_now_add

automatically set field when object first created. useful creation of timestamps. note current date used; it’s not default value can override.

if want have default time value of 'last_version_date' use 'default' key timezone.now function input

from django.utils import timezone class poll(models.model):     ....     last_version_date = models.datetimefield(default=timezone.now,blank=false)  

see answer. save function can become

def save(self, *args, **kwargs):      keep_history = kwargs.get('keep_history', false)        if keep_history:         self.last_version_date = self.modified         self.version = self.version + 1 

and use following

>>> p.save(keep_history=true) 

you can use, concurrency, integerversionfield version field.