PrimaryKeyRelatedField with 'source' mapping to nullable foreign key field results in error while serializing

[osumoclement]

I am encountering a problem that's related to issue #5750
In my case, I am trying to use PrimaryKeyRelatedField which doesn't work. IntegerField does work, but I really want to use PrimaryKeyRelatedField for consistency with the rest of my code:

#this doesn't work
foreign_field_attribute = serializers.PrimaryKeyRelatedField(source='foreign_field.my_attribute', allow_null=True, required=False)

#this works
foreign_field_attribute = serializers.IntegerField(source='foreign_field.my_attribute.id', allow_null=True, required=False)

As this is almost identical to issue #5750, I think the solution would be to apply the code that cleared issue up for IntegerField, CharField etc to PrimaryKeyRelatedField.

UPDATE
The problem occurs when the object is created (i.e POST) and doesn't happen if it's a GET request. Even more strange is that at that point, serializer.is_valid() is true, but when you try to access serializer.data the error happens

The full debug log for this:

KeyError: 'foreign_field'
Internal Server Error: /api/stuff
Traceback (most recent call last):
  File "/home/user/Envs/project/lib/python3.11/site-packages/django/core/handlers/exception.py", line 55, in inner
    response = get_response(request)
               ^^^^^^^^^^^^^^^^^^^^^
File "/home/user/Envs/project/lib/python3.11/site-packages/django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
File "/home/user/Envs/project/lib/python3.11/site-packages/django/views/decorators/csrf.py", line 56, in wrapper_view
    return view_func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/django/views/generic/base.py", line 104, in view
    return self.dispatch(request, *args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/run/media/user/drive/project/Django/stuff/views.py", line 152, in post
    "data": serializer.data,
            ^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/serializers.py", line 555, in data
    ret = super().data
          ^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/serializers.py", line 255, in data
    self._data = self.to_representation(self.validated_data)
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/serializers.py", line 509, in to_representation
    attribute = field.get_attribute(instance)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/relations.py", line 174, in get_attribute
    attribute_instance = get_attribute(instance, self.source_attrs[:-1])
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/user/Envs/project/lib/python3.11/site-packages/rest_framework/fields.py", line 94, in get_attribute
    instance = instance[attr]