[Solved]-How to access serializer.data on ListSerializer parent class in DRF?


At the point in the trace where I try to access serializer.data and get the KeyError, I note that serializer.data only contains key/vaule pairs from the initial_data, not the instance data (hence, I suppose, the KeyError; some model fields’ keys are not present as it is a partial_update request). However, serializer.child.data does contain all the instance data for the last child in the list.

So, I go to the rest_framework/serializers.py source where data is defined:

249    @property
250    def data(self):
251        if hasattr(self, 'initial_data') and not hasattr(self, '_validated_data'):
252            msg = (
253                'When a serializer is passed a `data` keyword argument you '
254                'must call `.is_valid()` before attempting to access the '
255                'serialized `.data` representation.\n'
256                'You should either call `.is_valid()` first, '
257                'or access `.initial_data` instead.'
258            )
259            raise AssertionError(msg)
261        if not hasattr(self, '_data'):
262            if self.instance is not None and not getattr(self, '_errors', None):
263                self._data = self.to_representation(self.instance)
264            elif hasattr(self, '_validated_data') and not getattr(self, '_errors', None):
265                self._data = self.to_representation(self.validated_data)
266            else:
267                self._data = self.get_initial()
268        return self._data

Line 265 is problematic. I can replicate the error by calling serializer.child.to_representation({'uuid': '87956604-fbcb-4244-bda3-9e39075d510a', 'product_code': 'foobar'}) at the breakpoint.

Calling partial_update() works fine on a single instance (because self.instance is set, self.to_representation(self.instance) works). However, for a bulk partial_update() implementation, self.validated_data is missing model fields, and to_representation() won’t work, so I won’t be able to access the .data property.

One option would be to maintain some sort of self.instances list of Product instances, and override the definition of data on line 265:

self._data = self.to_representation(self.instances)

I’d really prefer an answer from someone more experienced in this sort of problem though, as I’m not sure if that’s a sensible solution, hence I’m leaving the bounty open in the hope that someone can suggest something smarter to do.



As mentioned in the comment i still think the exception could be because of the user field in BulkProductSerializer class, not really anything to do with ListSerializer

There might be another minor error (but important) in the serializer DRF as mentioned in the documentation here. Here is how to specify a list_serializer_class:

class CustomListSerializer(serializers.ListSerializer):

class CustomSerializer(serializers.Serializer):
    class Meta:
        list_serializer_class = CustomListSerializer

Note that it’s specified inside of the Meta class, not outside. So i think in your code, it will not understand to switch to the List Serializer with many=True. That should cause the not-updating problem.

Update – Add example for updating nested list serializer

It seems that the question was more about a generic way to implement updating for nested List Serializer rather than the actual error. Therefore, i will try to provide a sample code.

Some notes:

  • If we use ModelViewSet, the list route won’t allow PUT or PATCH, so neither update nor partial_update will be called (reference). Therefore i use POST directly, this is much simpler.
  • If you want to use PUT/PATCH, then see this answer here
  • We can always add a query parameter like allow_update or partial directly to the Post request to differentiate between POST/PUT/PATCH
  • Instead of using uuid like the question i will use the normal id, it should be very much the same

It was rather simple

For reference, models look like this:

class Product(models.Model):
    name = models.CharField(max_length=200)
    user = models.ForeignKey(User, null=True, blank=True)

    def __unicode__(self):
        return self.name

Step 1: Make sure serializer change to ListSerializer

class ProductViewSet(viewsets.ModelViewSet):
    serializer_class = ProductSerializer
    queryset = Product.objects.all()

    def get_serializer(self, *args, **kwargs):
        # checking for post only so that 'get' won't be affected
        if self.request.method.lower() == 'post':
            data = kwargs.get('data')
            kwargs['many'] = isinstance(data, list)
        return super(ProductViewSet, self).get_serializer(*args, **kwargs)

Step 2: Implement the ListSerializer by overriding create function

class ProductListSerializer(serializers.ListSerializer):
    def create(self, validated_data):
        new_products = [Product(**p) for p in validated_data if not p.get('id')]
        updating_data = {p.get('id'): p for p in validated_data if p.get('id')}
        # query old products
        old_products = Product.objects.filter(id__in=updating_data.keys())
        with transaction.atomic():
            # create new products
            all_products = Product.objects.bulk_create(new_products)
            # update old products
            for p in old_products:
                data = updating_data.get(p.id, {})
                # pop id to remove
                updated_p = Product(id=p.id, **data)
        return all_products

class ProductSerializer(serializers.ModelSerializer):
    user = serializers.SlugRelatedField(slug_field='username', queryset=User.objects.all())
    id = serializers.IntegerField(required=False)

    class Meta:
        model = Product
        fields = '__all__'
        list_serializer_class = ProductListSerializer


Your Error has nothing to do with ListSerializer, but a problem getting field user:

KeyError: "Got KeyError when attempting to get a value for field user on serializer BulkProductSerializer.

The serializer field might be named incorrectly and not match any attribute or key on the OrderedDict instance.

Original exception text was: ‘fk_user’."

Make sure your Product model has a fk_user field.

You also have defined user field on BulkProductSerializer as writable but have not told the serializer how to handle it…

The easiest way to correct this is to use a SlugRelatedField:

class BulkProductSerializer(serializers.ModelSerializer):

    list_serializer_class = CustomProductListSerializer

    user = serializers.SlugRelatedField(

    class Meta:
        model = Product
        fields = (

This should handle nicely errors, for example when username does not exist…


Remove source if you are using Django auth model and set read_only=True.

user = serializers.CharField(read_only=True)

Hope this works for you


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