AssertionError on model_field with m2m

[lerela]

Describe the bug
Sorry @tfranzel for spamming the bug reports! This looks like a regression in 0.13.2 (also in master).

If I have a simple m2m relationship:

class Reltd(models.Model):
    test = models.CharField(max_length=50)
    testm = models.ManyToManyField("TestModel")

class TestModel(models.Model):
    test_field = models.CharField(max_length=50)

and I try to serialize the m2m relationship (without any django-filter involved):

class TestSerializer(serializers.ModelSerializer):
    mult = serializers.PrimaryKeyRelatedField(
        many=True, read_only=True, source="reltd_set"
    )

    class Meta:
        model = TestModel
        fields = ['id', 'mult']

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

Then I'm getting a strange warning and an AssertionError:

Warning #1: could not resolve field on model <class 'test_spectacular.models.TestModel'> with path "reltd_set". this is likely a custom field that does some unknown magic. maybe consider annotating the field/property? defaulting to "string". (Exception: TestModel has no field named 'reltd_set')
Traceback (most recent call last):
  File "./manage.py", line 22, in <module>
    main()
  File "./manage.py", line 18, in main
    execute_from_command_line(sys.argv)
  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 83, in get_operation
    operation['responses'] = self._get_response_bodies()
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 969, in _get_response_bodies
    return {'200': self._get_response_for_code(response_serializers, '200')}
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 1007, in _get_response_for_code
    component = self.resolve_serializer(serializer, 'response')
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 1146, in resolve_serializer
    component.schema = self._map_serializer(serializer, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 669, in _map_serializer
    schema = self._map_basic_serializer(serializer, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 736, in _map_basic_serializer
    schema = self._map_serializer_field(field, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 488, 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 516, in _map_serializer_field
    schema = self._map_model_field(model_field, direction)
  File "test_spectacular/venv/lib/python3.8/site-packages/drf_spectacular/openapi.py", line 387, in _map_model_field
    assert isinstance(model_field, models.Field)
AssertionError

However this works if I do not use read_only=True and provide a queryset.

Also, this works as expected with 0.13.1. Error with 0.13.2 is different from master (django.core.exceptions.FieldDoesNotExist: TestModel has no field named 'reltd_set').

[tfranzel]

@lerela all good, i'm glad someone is testing this before i release 0.14.0.

in 0.13.1 there was a bug here that threw no errors, but only produced a correct schema by accident.
in 0.13.2 there was a bugfix that corrected this but broke relations. (#274)
on master i attempted to fix the broken relation while retaining the correct solution (df6ef05)

i guess we are not done yet. these 10 lines are a whack-a-mole.

[tfranzel]

fixed several inconsistencies that lead to problems. this includes 2 unreported "bugs" that hid behind this issue. it basically comes down to unhandled DRF internal mechanics and an under-specified interface to follow_field_source.

its a hefty change, so please check for differences

[lerela]

This seems to fix the issue and I have not seen side effects − but my use cases are not so intricate. Well done 👍