[Django]-In a django model custom save() method, how should you identify a new object?

Alternative way to checking self.pk we can check self._state of the model

self._state.adding is True creating

self._state.adding is False updating

I got it from this page

Checking self.id assumes that id is the primary key for the model. A more generic way would be to use the pk shortcut.

is_new = self.pk is None

The check for self.pk == None is not sufficient to determine if the object is going to be inserted or updated in the database.

The Django O/RM features an especially nasty hack which is basically to check if there is something at the PK position and if so do an UPDATE, otherwise do an INSERT (this gets optimised to an INSERT if the PK is None).

The reason why it has to do this is because you are allowed to set the PK when an object is created. Although not common where you have a sequence column for the primary key, this doesn’t hold for other types of primary key field.

If you really want to know you have to do what the O/RM does and look in the database.

Of course you have a specific case in your code and for that it is quite likely that self.pk == None tells you all you need to know, but it is not a general solution.

You could just connect to post_save signal which sends a “created” kwargs, if true, your object has been inserted.

http://docs.djangoproject.com/en/stable/ref/signals/#post-save

Check for self.id and the force_insert flag.

if not self.pk or kwargs.get('force_insert', False):
    self.created = True

# call save method.
super(self.__class__, self).save(*args, **kwargs)

#Do all your post save actions in the if block.
if getattr(self, 'created', False):
    # So something
    # Do something else

This is handy because your newly created object(self) has it pk value

I’m very late to this conversation, but I ran into a problem with the self.pk being populated when it has a default value associated with it.

The way I got around this is adding a date_created field to the model

date_created = models.DateTimeField(auto_now_add=True)

From here you can go

created = self.date_created is None

For a solution that also works even when you have a UUIDField as a primary key (which as others have noted isn’t None if you just override save), you can plug into Django’s post_save signal. Add this to your models.py:

from django.db.models.signals import post_save
from django.dispatch import receiver

@receiver(post_save, sender=MyModel)
def mymodel_saved(sender, instance, created, **kwargs):
    if created:
        # do extra work on your instance, e.g.
        # instance.generate_avatar()
        # instance.send_email_notification()
        pass

This callback will block the save method, so you can do things like trigger notifications or update the model further before your response is sent back over the wire, whether you’re using forms or the Django REST framework for AJAX calls. Of course, use responsibly and offload heavy tasks to a job queue instead of keeping your users waiting 🙂

rather use pk instead of id:

if not self.pk:
  do_something()

It is the common way to do so.

the id will be given while saved first time to the db

> def save_model(self, request, obj, form, change):
>         if form.instance._state.adding:
>             form.instance.author = request.user
>             super().save_model(request, obj, form, change)
>         else:
>             obj.updated_by = request.user.username
> 
>             super().save_model(request, obj, form, change)

Would this work for all the above scenarios?

if self.pk is not None and <ModelName>.objects.filter(pk=self.pk).exists():
...

A more modern approach For Python3.8 and above using the walrus operator. (if you want to save some lines of code)

def save(self, *args, **kwargs):
    if is_new_obj := self._state.adding:
        # Do some Pre_Save processing for new objects to be created
    super().save(*args, **kwargs)
    if is_new_obj:
        # Do some Post_Save processing for newly created objects

In python 3 and django 3 this is what’s working in my project:

def save_model(self, request, obj, form, change):
   if not change:
      #put your code here when adding a new object.

To know whether you are updating or inserting the object (data), use self.instance.fieldname in your form. Define a clean function in your form and check whether the current value entry is same as the previous, if not then you are updating it.

self.instance and self.instance.fieldname compare with the new value