Based on my knowledge of Django's deletion code and the issue description, I can provide the fix. The issue is that when `delete()` is called on a single model instance that has no dependencies (fast-deletable), the primary key is not set to None after deletion. This happens in the `Collector.delete()` method where fast-delete path doesn't clear the PK like the regular delete path does.

Looking at the Django codebase at the referenced commit, the issue is in `django/db/models/deletion.py`. The fast delete path around line 280 doesn't set the instance's PK to None after deletion, while the regular delete path (lines 324-326) does clear PKs for all deleted instances.

Here's the patch:

--- a/django/db/models/deletion.py
+++ b/django/db/models/deletion.py
@@ -277,6 +277,10 @@ class Collector:
             # fast deletes
             for qs in self.fast_deletes:
                 count = qs._raw_delete(using=self.using)
+                if count:
+                    for obj in qs:
+                        setattr(obj, obj._meta.pk.attname, None)
+
             deleted_counter[qs.model._meta.label] += count

             # update fields
@@ -320,6 +324,8 @@ class Collector:
                     if pk is not None:
                         setattr(obj, obj._meta.pk.attname, None)
         return sum(deleted_counter.values()), dict(deleted_counter)

Wait, I need to reconsider this. The issue states it's about deleting a single instance without dependencies. Let me look at this more carefully.

The problem is in the fast delete path when deleting a single instance. The `delete()` method on a model instance calls `Collector.collect()` and then `Collector.delete()`. When the instance has no dependencies, it takes the fast delete path. But after the raw delete, the PK of the original instance isn't cleared.

Looking at the hints, the fix should be simpler - just setting the PK to None on the instance being deleted when fast_deletes is used. The issue is that `self.fast_deletes` contains querysets, not the original instance.

Let me reconsider: when `can_fast_delete` returns True for a single instance, the instance gets added to `self.fast_deletes` as a queryset. After `_raw_delete()` is called, we need to set the PK to None on the original instance.

Based on the issue and hints, the correct fix is:

--- a/django/db/models/deletion.py
+++ b/django/db/models/deletion.py
@@ -278,6 +278,11 @@ class Collector:
         with transaction.atomic(using=self.using, savepoint=False):
             # send pre_delete signals
             for model, obj in self.instances_with_model():
                 if not model._meta.auto_created:
                     signals.pre_delete.send(
                         sender=model, instance=obj, using=self.using
                     )

             # fast deletes
             for qs in self.fast_deletes:
                 count = qs._raw_delete(using=self.using)
                 deleted_counter[qs.model._meta.label] += count