Django deleting where SET_NULL - django

Question

I have an issue with a ForeignKey and on_delete=SET_NULL.
When deleting the last_data referenced in my Stuff model, it also deletes the stuff object just as if this was a cascade, which is obviously not what I expected, rather setting the last_data field null.
Here is how my models are defined in two different django apps.
# App 1
class Device(models.Model):
last_data = models.ForeignKey('Data', null=True, blank=True, related_name="last_data_device", on_delete=models.SET_NULL, help_text="Latest data")
class Data(models.Model):
content = models.CharField(max_length=255)
date_created = models.DateTimeField(blank=True, null=False, db_index=True)
device = models.ForeignKey(Device, related_name="data", db_index=True, on_delete=models.CASCADE)
# App 2
class Stuff(models.Model):
device = models.OneToOneField(Device, null=True, blank=True, related_name="stuff", db_index=True, on_delete=models.CASCADE)
last_data = models.ForeignKey(Data, null=True, blank=True, help_text="Latest data", db_index=True, on_delete=models.SET_NULL)
I must have misunderstood how this is linked, what I want is that a stuff object is never deleted when data is removed, but that the last_data reference it has may be nulled when this happens.
How should I write this or what did I do wrong here?
Thanks
PS: Migrations are up to date and db is synced.
Well, seing the answers given, already, it seems I should clarify.
When I do :
>>> stuff = Stuff.objects.get(...)
>>> stuff.last_data.delete()
Is that this is the stuff object that gets removed as "dependancy" and I cannot understand why.
What I'd expect is that the last_data field gets nulled and the stuff object is left alone.

Answer 1

Maybe this is due to communication OneToOneField. If you delete your last_data referenced in your Stuff model, where device connet OneToOne with Device. You can set on_delete=models.SET_NULL on your OneToOne field

Answer 2

Change to models.DO_NOTHING. That would avoid the delete.

Answer 3

If I understood correctly, your problem is that when you do:
>>> stuff = Stuff.objects.get(...)
>>> stuff.last_data.delete()
then the corresponding Data record gets removed from the database as well.
This is expected, and it's how it supposed to work. If all you want to do is to set last_data to NULL, while keeping the Data table intact, then:
>>> stuff.last_data = None
>>> stuff.save()
The purpose of on_delete=SET_NULL is to support the opposite use case: when you delete a Data record, then all Stuff records that were pointing to that Data will get their last_data set to NULL:
>>> stuff = Stuff.objects.get(...)
>>> stuff.last_data
<Data: ...>
>>> Data.objects.all().delete()
>>> stuff.last_data
None