class A(models.Model):
    title = models.TextField()

class B(models.Model)
    a = models.ForeignKey(related_name='b_objects', on_delete=models.CASCADE)

class BSerializer(serializers.ModelSerializer):

    class Meta:
        model = B
        fields = ('id', 'a')

class ItemViewSet(generics.GenericAPIView):
    serializer_class = BSerializer
    queryset = B.objects.all()

    def post(self, *args, **kwargs):
        # Если передать ему не существующий ключ например 100 с параметром many True
        serializer = self.get_serializer(data=[{'a': 100}], many=True)
        serializer.is_valid(raise_exception=True)
        return Response({"test": 'test_value'}, status=status.HTTP_201_CREATED

#  File "lib/python3.7/site-packages/rest_framework/relations.py", line 272, #in to_representation
#    return value.pk
#AttributeError: 'int' object has no attribute 'pk'