ModelForm primary_key update with instance set is executed as INSERT caused by is_valid() changing the instance

[amin@…]

I do agree this is a bad database design but it's executed without any warning or anything in the docs so i want to open this ticket to allow the discussion of raising an Error or anything (e.g. an info in the docs @primary_key or modelform) not let anyone run into this mistake.

class Test(models.Model):
  email = models.CharField(max_length=100, primary_key=True)

class TestForm(forms.ModelForm):
  class Meta:
     model = Test 

# assuming this object exists
x = Test.objects.get(email='old@example.com')

form = TestForm(email='new@example.com', instance=x)

# here instance is updated with new@example.com as pkey and django thinks it has to create a new model instance not updating the old instance

if form.is_valid():
   new_x = form.save()
   new_x.save()

[Karen Tracey]

A concrete suggestion for what's wanted here would help, as it is I'm not sure what's being asked for.