Fixes: #558 - Avoid "<type> None" in CustomObject.__str__ post-delete

[Kani999]

Fixes: #558

Summary

CustomObject.__str__ includes self.id in its fallback output when no field on the COT is marked primary=True. But Django's Model.delete() sets self.pk to None after the database row is removed — and NetBox's delete-toast and changelog renderers call str() on the post-delete instance. The result is a literal "None" in the user-facing message:

Deleted SmokeTabTarget SmokeTabTarget None

Fix

Branch on self.pk inside the no-primary-field path:

• self.pk is None (post-delete) → return just the COT's display name ("SmokeTabTarget").
• self.pk is set (normal rendering) → behave as before ("SmokeTabTarget 5").

8 lines, no behavioural change for any code path with a live pk.

Verification

Observed while smoke-testing PR #482's related-tabs work.

Context

Surfaced during the related-object-tabs smoke testing (PR #482); independent of that work and easy enough to land standalone.

[bctiemann]

Hi @Kani999 , as with the other PR just now, please don't open a PR unless there is an accepted bug to link it to (this is for later bookkeeping/forensics). Please open a bug issue with repro steps and any relevant details according to the template, and then once it's triaged and accepted we can reopen this PR. Thanks!

[bctiemann]

Two small suggestions:

1. Trim the comment (project convention is one short line max):

if self.pk is None:  # post-delete: pk is cleared by Django's Model.delete()
    return str(self.custom_object_type.display_name)

2. Regression test in PrimaryFieldChangeTestCase in tests/test_field_types.py:

def test_str_post_delete_no_primary_field_does_not_contain_none(self):
    """Regression #558: str() on a post-delete instance (pk=None, no primary
    field value) must return the COT display name, not '<type> None'."""
    cot = self.create_custom_object_type(
        name="DeleteStrTest", slug="delete-str-test",
        verbose_name_plural="Delete Str Tests",
    )
    # No primary field — falls through to the pk fallback path in __str__.
    model = cot.get_model()
    instance = model.objects.create()

    instance.delete()  # sets instance.pk = None

    result = str(instance)
    self.assertNotIn("None", result)
    self.assertEqual(result, cot.display_name)

Both verified locally — test passes with the fix and fails on the unpatched code.

[bctiemann]

See above