TypeError: 'RelatedManager' object is not iterable

[donspaulding]

Given some customized Serializer and HRF fields, as follows:

class RelativeHyperlinkedRelatedField(HyperlinkedRelatedField):
    def get_url(self, obj, view_name, request, format):
        lookup_field = getattr(obj, self.lookup_field)
        kwargs = {self.lookup_field: lookup_field}
        return reverse(view_name, kwargs=kwargs, request=None, format=format)

    def get_identity(self, data):
        try:
            return data.get('resource_uri', None)
        except AttributeError:
            return None


class PCPModelSerializer(HyperlinkedModelSerializer):
    """
    Adds both a relative and absolute URI field on all serialized instances.
    """
    _hyperlink_field_class = RelativeHyperlinkedRelatedField  # Controls the nature of related URIs
    field_mapping = HyperlinkedModelSerializer.field_mapping.copy()

    def get_identity(self, data):
        try:
            return data.get('resource_uri', None)
        except AttributeError:
            return None

    def get_default_fields(self):
        """
        Customize our URI fields.
        """
        fields = super(HyperlinkedModelSerializer, self).get_default_fields()

        if self.opts.view_name is None:
            self.opts.view_name = self._get_default_view_name(self.opts.model)

        uri_fields = {
            'resource_uri': RelativeHyperlinkedIdentityField(
                view_name=self.opts.view_name,
                lookup_field=self.opts.lookup_field
            ),
            'absolute_uri': HyperlinkedIdentityField(
                view_name=self.opts.view_name,
                lookup_field=self.opts.lookup_field
            )
        }
        for uri_field_name, uri_field in uri_fields.items():
            if uri_field_name not in fields:
                fields.insert(0, uri_field_name, uri_field)
        return fields


def partially_nested(serializer_class, serializer_kwargs=None, include_fields=None, exclude_fields=None):
    serializer_kwargs = serializer_kwargs or {}
    partial_fields = set(include_fields or []) - set(exclude_fields or [])
    default_fields = set(serializer_class(**serializer_kwargs).fields)
    excluded_fields = tuple(default_fields - partial_fields)

    def get_fields(self, *args, **kwargs):
        fields = super(self.__class__, self).get_fields(*args, **kwargs)
        for field_name in excluded_fields:
            fields.pop(field_name, None)
        return fields
    new_serializer_class = type(
        serializer_class.__name__,  # New class name
        (serializer_class,),  # New class bases (must be a tuple)
        {'get_fields': get_fields},   # New class attributes
    )
    return new_serializer_class(**serializer_kwargs)

And the following serializer definitions:

class AttachmentSerializer(PCPModelSerializer):
    class Meta:
        model = Attachment
        fields = ('payment')

class PaymentSerializer(PCPModelSerializer):
    attachments = partially_nested(
        AttachmentSerializer,
        include_fields=['resource_uri', 'absolute_uri']
    )
    class Meta:
        model = Payment

And when attempting to PUT a Payment object back to the /api/payments/ endpoint (which is a ModelViewSet pointing to PaymentSerializer as its class), like so:

{
  "resource_uri": "/api/payments/12/",
  "attachments": [
        {"resource_uri": "/api/attachments/43/"},
        {"resource_uri": "/api/attachments/44/"}
  ]
}

I get the following traceback:

Traceback (most recent call last):
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = callback(request, *callback_args, **callback_kwargs)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/viewsets.py", line 78, in view
    return self.dispatch(request, *args, **kwargs)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 77, in wrapped_view
    return view_func(*args, **kwargs)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/views.py", line 327, in dispatch
    response = self.handle_exception(exc)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/views.py", line 324, in dispatch
    response = handler(request, *args, **kwargs)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/mixins.py", line 129, in update
    if serializer.is_valid():
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 479, in is_valid
    return not self.errors
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 471, in errors
    ret = self.from_native(data, files)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 867, in from_native
    instance = super(ModelSerializer, self).from_native(data, files)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 317, in from_native
    attrs = self.restore_fields(data, files)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 243, in restore_fields
    field.field_from_native(data, files, field_name, reverted_data)
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 398, in field_from_native
    if serializer.is_valid():
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 479, in is_valid
    return not self.errors
  File "/Users/don/Development/python/virtualenvs/pcp/lib/python2.7/site-packages/rest_framework/serializers.py", line 447, in errors
    identities = [self.get_identity(self.to_native(obj)) for obj in objects]
TypeError: 'RelatedManager' object is not iterable

objects in that bottom frame is the Django Payment model's "attachments" reverse related manager.

Any idea what I'm not supplying to DRF so that it knows to use Payment.attachments.all() or something similar?

Thanks,
Don Spaulding

[lovelydinosaur]

Any chance you could try to break this down to something much smaller that replicates the issue.

Note that there's something similar in the relations code, here... https://github.com/tomchristie/django-rest-framework/blob/master/rest_framework/relations.py#L147

Might be worth referencing the history of those lines to see what the test case associated with the relationship case looks like.

[dulacp]

Quite sure that it's the same as #960 (were I used a minimalist example), since the error occurred like mine on a PUT request with nested objects.
You may want to try to integrate the #750 pull request commits since it fixed the issue on my side.

Cheers

[donspaulding]

Thanks @dulaccc! That does in fact appear to be the issue.

I'm fine with this issue being closed, however, it seems that the "Note" at the bottom of the docs on nested serializers should be in a red-background warning box at the top of this section: http://django-rest-framework.org/api-guide/serializers.html#dealing-with-nested-objects

It's my own fault for not fully reading the docs, but I just assumed that nesting a model serializer would work for PUTs just like GETs. Having a more in-my-face warning would have saved me a lot of time last week.

Thanks again all.

[poswald]

This is closed and works ok for PUT but does it make sense that it's still not fixed for PATCH calls? I'm getting this with a serializer.is_valid() that has been initialized with partial=True. If you change that line to objects = self.object.all() it seems to work. If so, I can open it as a new issue but I'm not sure it makes sense with the impending 3.0 release.