Using SerializerMethodField with many=True on CreateView results in error

[matteing]

Checklist

• [x] I have verified that that issue exists against the master branch of Django REST framework.
• [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [x] This is not a usage question. (Those should be directed to the discussion group instead.)
• [x] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• [x] I have reduced the issue to the simplest possible case.
• [x] I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

When issuing a POST request from the Rest Framework UI to an endpoint that allows lists (many=True) to be created, if the serializer has a MethodField, the object passes as dictionary and errors with 'collections.OrderedDict'.

Background

class TaskSerializer(serializers.ModelSerializer):
    user = UserSerializer(
        read_only=True,
        default=serializers.CurrentUserDefault()
    )
    project_set = ProjectSerializer(many=True, read_only=True)
    praise = serializers.SerializerMethodField(read_only=True)
    user_praised = serializers.SerializerMethodField(read_only=True)

    def get_praise(self, obj):
        # BUG WORKAROUND
        # Hack otherwise breaks /smart/. I don't know why. Rest framework bug.
        if isinstance(obj, Task):
            return obj.praise_amount
        else:
            return 0

    def get_user_praised(self, obj):
       # Unmodified bug case
        request = self.context.get('request', False)
        if request and request.user.is_authenticated:
            return obj.user_praise_amount(request.user)
        else:
            return None

    @staticmethod
    def setup_eager_loading(queryset):
        """ Perform necessary eager loading of data. """
        # select_related for "to-one" relationships
        queryset = queryset.select_related('user')

        # prefetch_related for "to-many" relationships
        queryset = queryset.prefetch_related(
            'project_set',
            'project_set__user')

        return queryset

    class Meta:
        model = Task
        fields = (
            'id',
            'event',
            'done',
            'content',
            'created_at',
            'updated_at',
            'done_at',
            'user',
            'project_set',
            'praise',
            'user_praised',
            'attachment',
        )
        read_only_fields = (
            'event',
            'done_at',
        )

View code:

class SmartTaskView(CreateAPIView):
    serializer_class = TaskSerializer

    def create(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data, many=True)
        serializer.is_valid(raise_exception=True)
        self.perform_create(serializer)
        headers = self.get_success_headers(serializer.data)
        for t in serializer.data:
            process_task_tags(Task.objects.get(pk=t['id']), request.user)
        return Response(serializer.data, status=status.HTTP_201_CREATED, headers=headers)

Expected behavior

On regular JS post requests using axios and JS arrays, the POST request works as expected, adding everywhere from 1 to unlimited tasks. It returns 201 and the Task object with correct fields.

It is expected for the Rest Framework UI to do the same, yet it does not.

Working request body:

Actual behavior

An AttributeError occurs because a dict is passed to MethodField instead of an instance, the latter being expected behavior. This does not happen when using Many=True on GET views.

AttributeError at /smart/
'collections.OrderedDict' object has no attribute 'user_praise_amount'

[matteing]

Any help on this?

[rpkilby]

Hi @matteing, the full stack trace would be helpful in pinpointing where the exception is occurring.

[matteing]

Absolutely, I've included it below:

Environment:

Request Method: POST
Request URL: http://localhost:8000/smart/

Django Version: 2.0.1
Python Version: 3.6.5
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'rest_framework',
 'rest_framework.authtoken',
 'corsheaders',
 'django_q',
 'makerlog',
 'stats',
 'accounts',
 'pages',
 'projects',
 'integrations',
 'memoize',
 'django_extensions',
 'invites',
 'sensors',
 'anymail',
 'notifications',
 'discussions']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'corsheaders.middleware.CorsMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware',
 'makerlog.middleware.timezone.TimezoneMiddleware']

Traceback:

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  35.             response = get_response(request)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  158.                 response = self.process_exception_by_middleware(e, request)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  156.                 response = response.render()

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/django/template/response.py" in render
  106.             self.content = self.rendered_content

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/response.py" in rendered_content
  72.         ret = renderer.render(self.data, accepted_media_type, context)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/renderers.py" in render
  724.         context = self.get_context(data, accepted_media_type, renderer_context)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/renderers.py" in get_context
  697.             'post_form': self.get_rendered_html_form(data, view, 'POST', request),

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/renderers.py" in get_rendered_html_form
  520.             return self.render_form_for_serializer(serializer)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/renderers.py" in render_form_for_serializer
  528.             serializer.data,

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/serializers.py" in data
  560.         ret = super(Serializer, self).data

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/serializers.py" in data
  264.                 self._data = self.to_representation(self.validated_data)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/serializers.py" in to_representation
  527.                 ret[field.field_name] = field.to_representation(attribute)

File "/Users/sergio/.virtualenvs/makerlog-server-B0FiQlpH/lib/python3.6/site-packages/rest_framework/fields.py" in to_representation
  1855.         return method(value)

File "/Users/sergio/Projects/makerlog-server/projects/serializers.py" in get_user_praised
  55.             return obj.user_praise_amount(request.user)

Exception Type: AttributeError at /smart/
Exception Value: 'collections.OrderedDict' object has no attribute 'user_praise_amount'

[rpkilby]

If you disable the many flag and create a single object, do you receive the same error?

[matteing]

@rpkilby Indeed, error does disappear - although it kills the desired functionality, which is posting arrays of objects from JS.

The error does not occur when posting through JS though, strangely. Only through the browsable interface.

[rpkilby]

Of course - I just wanted to do a quick sanity check that this wasn't a general usage issue, and was related to the many usage.

[matteing]

Any updates?

[tomchristie]

The error does not occur when posting through JS though, strangely. Only through the browsable interface.

Indeed. I guess the first thing to do here would be for us to figure out if it's always been an issue as long as we've supported bulk-create, or if the issue has been introduced along the way.

[tomchristie]

Any updates?

If anyone wants to help progress this, I'd suggest:

1. Reducing the issue down to the simplest possible case. (Single field. Ideally just Serializer rather than ModelSerializer.)
2. Figuring out which versions it applies to.

[carltongibson]

Looks to me like this could never have worked.

The Browsable API is rendering the form from the serialised data (which is a collections.OrderedDict) and not an object instance.

This kind of thing always happens with the Browsable API — we get to a certain complexity and we're asking it to do too much...

[bufpal]

I had same problem... so I had to seperate to two serializers and two views. One is for the get request(fields with SerializerMethodField), the other one is for the post request.