Model.get_cached() cache is not invalidated on ForeignKey update from on_delete

[philipkimmey]

Let's say I have a Person object with a ForeignKey to a default image.

class Person(models.Model):
    default_image = models.ForeignKey('people.Image', blank=True, null=True,
         on_delete=models.SET_NULL)

class Image(models.Model):
    some_image_field = models.ImageField(...)

>>> p = Person.objects.get(first_name='Phil')
>>> img = Image.objects.get(**some_kwargs)
>>> p_cached = Person.get_cached(pk=p.pk)
>>> p.default_image = img
>>> p.save()
>>> p_cached = Person.get_cached(pk=p.pk)
>>> p.default_image is not None
True
>>> p_cached.default_image is not None
True
>>> img.delete()
>>> p.default_image is None
True
>>> p_cached = Person.get_cached(pk=p.pk)
>>> p_cached.default_image is None
Traceback (most recent call last)
...
File "django/db/models/fields/related.py", line 316, in __get__
rel_obj = QuerySet(self.field.rel.to).using(db).get(**params)
DoesNotExist: Image matching query does not exist.

It's a pretty tricky bug, and it may be nearly impossible to fix if the on_delete actually sets a database property that leads to that behavior rather than doing it in Python.

Nonetheless, I felt I should probably report it just so it's a known issue.