Field 'id' expected a number but got error in serializing custom user model in Django rest framework code example

Example 1: how to add methods to manuplate the model in serializers

from rest_framework import serializers, status
from models import *


class UserSerializer(serializers.ModelSerializer):
    class Meta:
        model = User
        fields = ('username', 'first_name', 'last_name', 'email')


class StudentSerializer(serializers.ModelSerializer):
    """
    A student serializer to return the student details
    """
    user = UserSerializer(required=True)

    class Meta:
        model = UnivStudent
        fields = ('user', 'subject_major',)

    def create(self, validated_data):
        """
        Overriding the default create method of the Model serializer.
        :param validated_data: data containing all the details of student
        :return: returns a successfully created student record
        """
        user_data = validated_data.pop('user')
        user = UserSerializer.create(UserSerializer(), validated_data=user_data)
        student, created = UnivStudent.objects.update_or_create(user=user,
                            subject_major=validated_data.pop('subject_major'))
        return student

Example 2: how to add methods to manuplate the model in serializers

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

class UnivStudent(models.Model):
    """
    A class based model for storing the records of a university student
    Note: A OneToOne relation is established for each student with User model.
    """
    user = models.OneToOneField(User)
    subject_major = models.CharField(name="subject_major", max_length=60)