AttributeError: 'ListSerializer' object has no attribute 'Meta'

[treussart]

Describe the bug
I get this error:

Traceback (most recent call last):
  File "/Users/mtreussart/git/tt/scripts/../src/manage.py", line 21, in <module>
    main()
  File "/Users/mtreussart/git/tt/scripts/../src/manage.py", line 17, in main
    execute_from_command_line(sys.argv)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/django/core/management/__init__.py", line 419, in execute_from_command_line
    utility.execute()
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/django/core/management/__init__.py", line 413, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/django/core/management/base.py", line 354, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/django/core/management/base.py", line 398, in execute
    output = self.handle(*args, **options)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/management/commands/spectacular.py", line 56, in handle
    schema = generator.get_schema(request=None, public=True)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/generators.py", line 243, in get_schema
    paths=self.parse(request, public),
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/generators.py", line 217, in parse
    operation = view.schema.get_operation(
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 84, in get_operation
    operation['responses'] = self._get_response_bodies()
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 997, in _get_response_bodies
    return {'200': self._get_response_for_code(response_serializers, '200')}
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 1042, in _get_response_for_code
    component = self.resolve_serializer(serializer, 'response')
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 1184, in resolve_serializer
    component.schema = self._map_serializer(serializer, direction)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 687, in _map_serializer
    schema = self._map_basic_serializer(serializer, direction)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 756, in _map_basic_serializer
    schema = self._map_serializer_field(field, direction)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 483, in _map_serializer_field
    schema = self._map_serializer_field(field.child, direction, collect_meta)
  File "/Users/mtreussart/.local/share/virtualenvs/back-JrD3H-kk/lib/python3.9/site-packages/drf_spectacular/openapi.py", line 653, in _map_serializer_field
    target = follow_field_source(field.parent.Meta.model, field.source_attrs)
AttributeError: 'ListSerializer' object has no attribute 'Meta'

To Reproduce
This is the problematic field:

groups = serializers.ListSerializer(
    source="all_groups", child=UserGroupField(), required=False, read_only=True
)

For fix the problem I patched openapi.py line 649:

        if isinstance(field, serializers.ReadOnlyField):
            # direct source from the serializer
            assert field.source_attrs, f'ReadOnlyField "{field}" needs a proper source'
            if hasattr(field.parent, "Meta"):
                target = follow_field_source(field.parent.Meta.model, field.source_attrs)
            else:
                if isinstance(field.parent, serializers.ListSerializer):
                     return append_meta(build_array_type(build_basic_type(OpenApiTypes.OBJECT)), meta)
            if callable(target):
                schema = self._map_response_type_hint(target)
            elif isinstance(target, models.Field):
                schema = self._map_model_field(target, direction)
            else:
                assert False, f'ReadOnlyField target "{field}" must be property or model field'
            return append_meta(schema, meta)

Expected behavior
No error. and understand my read-only serializer.

[tfranzel]

hi @treussart, nice catch! would you like to do a separate PR for that "scim commit"? i'm not fully sold on the args PR but this i would like to include. if you don't care, i can cherry-pick the commit. i can add a regression test later on.

[tfranzel]

Hi @treussart, sry to keep you waiting. while constructing a test something simply didn't sit right with me. i'm sure your fix did it for you, but i believe its not entirely correct. also this wraps with an array one time too many.

i think the core issue is that this piece of code did not account for ModelSerializer being 2 steps removed instead of 1. you simply circumvented that by returning a generic response. i think we can do better.

please confirm these assumptions i made. it was not clear from the example:

• UserGroupField is a subclass of serializers.ReadOnlyField
• group is nested inside a ModelSerializer
• all_groups is a model property?

could you try the following snippet instead? i think this is likely the correct fix.

            ...
            assert field.source_attrs, f'ReadOnlyField "{field}" needs a proper source'

            if is_list_serializer(field.parent):
                field = field.parent

            target = follow_field_source(field.parent.Meta.model, field.source_attrs)

            if callable(target):
            ...

EDIT: i changed the snipped. my previous proposal had one remaining issue. it's even simpler now.

[treussart]

Hi, I test your code and there is a new warning: Warning #0: UserViewSet: UserSerializer: unable to resolve type hint for function "all_groups". Consider using a type hint or @extend_schema_field. Defaulting to string.

    GroupRelation = namedtuple("GroupRelation", "group, direct")

    @property
    def all_groups(self) -> Iterable[GroupRelation]:
        """
        Yields all the groups a user is into, and whether the relationship is direct or not.
        """
        seen = set()
        for group in self.groups.all():
            seen.add(group)
            yield GroupRelation(group=group, direct=True)

[tfranzel]

at least we are making progress 😄 i added some hint detection.

[treussart]

Hi, thanks for the correction, I rebased my branch on master and it's good for me.

[tfranzel]

awesome! i'll close this then.