Allow to clear object before deleting to avoid data loss because delete cascade behaviour

[Manuel Saelices]

Imaging a model like this:

class Master(models.Model):
    name = models.CharField(max_length=100)
        

class Slave(models.Model):
    name = models.CharField(max_length=100)
    master = models.ForeignKey(Master, null=True)

And this object creation:

>>> m1 = Master.objects.create(name="master1")
>>> s1 = Slave.objects.create(name="slave1", master=m1)

>>> m1.delete() # this will also delete s1

Django by default, delete in cascade all related objects... even with null=True.

If you want to avoid this, you can do this:

>>> m1.slave_set.clear()
>>> m1.delete()

This is ok, but it was wonderful if I can change behaviour to not delete cascade. You can think in admin site for example. If you want to delete a Category object you have previously to enter in maybe a hundred objects to set null by hand.

I propose a definition like this:

class Slave(models.Model):
    name = models.CharField(max_length=100)
    master = models.ForeignKey(Master, null=True, delete_cascade=False)

With delete_cascade=False (by default would be True), delete() method will clear this relation previously deletion.

I've attached a patch that implements this idea.

[Alex Gaynor]

Dupe of #10262