'NoneType' object has no attribute 'Meta' with django-filter and through m2m model

[lerela]

Describe the bug
Using django-filter backend with a through relationship fails with 0.13.2. Not sure that's the exact issue but this does not happen without the "through".

To Reproduce
Models:

class Reltd(models.Model):
    test = models.CharField(max_length=50)

class TestModel(models.Model):
    reltd = models.ManyToManyField(Reltd, through="TestModelReltd")

class TestModelReltd(models.Model):
    tm = models.ForeignKey(TestModel, on_delete=models.PROTECT)
    rl = models.ForeignKey(Reltd, on_delete=models.PROTECT)

Views:

class MyFilter(filters.FilterSet):
    reltd = filters.ModelMultipleChoiceFilter(
        field_name="reltd", label="reltd"
    )

    class Meta:
        model = TestModel
        fields = ["reltd"]

class TestViewSet(viewsets.ModelViewSet):
    queryset = TestModel.objects.all()
    serializer_class = TestSerializer

    filter_backends = [
        filters.DjangoFilterBackend,
    ]
    filter_class = MyFilter 

Error:

  File "test_spectacular/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 401, in execute_from_command_line
    utility.execute()
  File "test_spectacular/venv/lib/python3.8/site-packages/django/core/management/__init__.py", line 395, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "test_spectacular/venv/lib/python3.8/site-packages/django/core/management/base.py", line 330, in run_from_argv
    self.execute(*args, **cmd_options)
  File "test_spectacular/venv/lib/python3.8/site-packages/django/core/management/base.py", line 371, in execute
    output = self.handle(*args, **options)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/management/commands/spectacular.py", line 50, in handle
    schema = generator.get_schema(request=None, public=True)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/generators.py", line 219, in get_schema
    paths=self.parse(request, public),
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/generators.py", line 196, in parse
    operation = view.schema.get_operation(path, path_regex, method, self.registry)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 61, in get_operation
    parameters = self._get_parameters()
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 190, in _get_parameters
    **dict_helper(self._get_filter_parameters()),
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 365, in _get_filter_parameters
    parameters += filter_extension.get_schema_operation_parameters(self)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/contrib/django_filters.py", line 43, in get_schema_operation_parameters
    result += self.resolve_filter_field(
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/contrib/django_filters.py", line 95, in resolve_filter_field
    schema = auto_schema._map_model_field(model_field, direction=None)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 407, in _map_model_field
    return self._map_serializer_field(field, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 480, in _map_serializer_field
    schema = self._map_serializer_field(field.child_relation, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 494, in _map_serializer_field
    model = field.parent.parent.Meta.model
AttributeError: 'NoneType' object has no attribute 'Meta'

This happens in 0.12, 0.13.0, 0.13.1 & 0.13.2 as field.parent.parent seems to be None in this case.

Expected behavior
No issue.

[tfranzel]

@lerela indeed this was a very special case that surfaced nowhere else. i far exceeded the time i have for django-filter because the library keeps on generating more edge cases. i opted to catch errors from now on to not break schema generation because of django-filter.

[lerela]

That makes total sense. Sane fallbacks are totally fine! Thanks again for all your great work (this solves that issue).