How do I access User Model field in the Serializer of extended User Model in Django Rest Framework?

I am creating a simple registration/login app in Django Rest Framework using Rest APIs with Django Rest Knox's authentication, in which, upon registration, user will also upload a CSV File (which is an optional field).

I have a Person Model which is an extended (derived or OneToOne) Model of User Model and has FileField for the file to be uploaded.

I have two ModelSerializers, one is based on User Model named MainRegisterSerializer and serializes username, email and password fields, and the other is based on Person Model named RegisterSerializer, which serializes FileField and register field (that is initialized with MainRegisterSerializer class's constructor.

Now, in views.py, I am accessing the RegisterSerializer, to get the data from front that which includes username, email and password fields from User Model, and FileField from Person Model.

But, when I enter data in front-end and upload the file, and click on POST button, I get the following error:

KeyError at /api/v1/register
'username'

on the code line user = User.objects.create_user(validated_data['username'], validated_data['email'], validated_data['password'], validated_data['file']) in serializers.py file.

My views.py is:

from rest_framework.response import Response
from knox.models import AuthToken
from .serializers import RegisterSerializer


class RegisterAPI(generics.GenericAPIView):
    serializer_class = RegisterSerializer

    def post(self, request, *args, **kwargs):
        serializer = self.get_serializer(data=request.data)
        serializer.is_valid(raise_exception=True)
        user = serializer.save()
        token = AuthToken.objects.create(user)
        return Response({
            "users": UserSerializer(user, context=self.get_serializer_context()).data,
            "token": token[1]
        })

My serializers.py is:

from .models import Person
from django.contrib.auth.models import User
from rest_framework import serializers


class MainRegisterSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('username', 'email', 'password')


class RegisterSerializer(serializers.ModelSerializer):
    register = MainRegisterSerializer()

    class Meta:
        model = Person
        fields = ['register', 'file',]
        optional_fields = ['file',]
        extra_kwargs = {'password': {'write_only': True}}

    def create(self, validated_data):
        user = User.objects.create_user(validated_data['username'], validated_data['email'], validated_data['password'], validated_data['file'])
        return user

My models.py is:

from django.db import models.
from django.contrib.auth.models import User


class Person(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    file = models.FileField(verbose_name="CSV File")

I have already worked around by trying various solutions told here on stackoverflow, but none of them worked for me, that's why I am posting the question. Any help will surely be appreciated. Thanks.

If you look at the validated data you can see, the data belonging to MainRegisterSerializer serializer will be available with register key. And data belonging to RegisterSerializer serializer will be available with file key.

validated data :

{'register': OrderedDict([('username', 'test'), ('email', 'test@gmail.com'), ('password', 'test123')]), 'file': <InMemoryUploadedFile: filename.txt (text/csv)>}

so to access any fields for register you need to use:

validated_data['register']['username'], validated_data['register']['email'] ..etc.

for accessing the file you can use : validated_data['file']