How can I update a Django models.DateTimeField when another, specific field in the same model is updated?

One way to do it is to define a save() method on your custom model looking for change in is_active. There is no easy way to achieve this: you need to manually save the previous state of the value of is_active by defining a custom __init__-method. It could look something like this:

class MyModel(models.Model): is_active = models.BooleanField(default=False) active_from = models.DateTimeField(blank=True) def __init__(self, *args, **kwargs): super().__init__(*args, **kwargs) self.__is_active = self.active def save(self, *args, **kwargs): if self.is_active and not self.__is_active: self.active_from = datetime.now() super().save(*args, **kwargs) 

This question has some answers which might help.

This is what I've pieced together so far. It seems like the most Django-esque way of doing this, but I'm happy to be corrected if not.

from django.db.models.signals import pre_save, post_save from django.dispatch import receiver import datetime class MyModel(models.Model): is_active = models.BooleanField(default=False) active_from = models.DateTimeField(blank=True) # Set active_from if active is set in an update @receiver(pre_save, sender=MyModel) def set_active_from_on_update(sender, instance, update_fields, **kwargs): if 'is_active' in update_fields and instance.is_active is True: instance.active_from = datetime.now() # Set active_from if active is set on create @receiver(post_save, sender=MyModel) def set_active_from_on_create(sender, instance, created, **kwargs): if created and instance.is_active is True: instance.active_from = datetime.now() 

My reasoning: update_fields in pre_save seems like the right place for any logic based on particular fields updating, but pre_save doesn't know if instance will be a new entry in the database or not, so post_save is needed to use the create boolean.

I think I could also do away with is_active and set active_from to null when it isn't active, but that doesn't seem as safe.

When you toggle is_active update active_from at the same time.

for example:

def toggle_user_active_and_update(request, *a, **kw): request.user.is_active = !request.user.is_active request.user.active_from = datetime.datetime.now() request.user.save() 

Another alternative is the FieldTracker from django-model-utils. I'm going to be using this moving forwards, as it makes more complex manipulation easier during save().