DRF getting AttributeError while trying to post data

This error occurs because there is no phone_number, date_of_birth, and address belong to User so DRF couldnt access to them. But when you putting write_only=True, DRF doesn't try to access them anymore, that's why it seems like write_only fixes the problem.

You could create nester serialiser ProfileSerializer and use it in your UserRegistrationSerializer:

class ProfileSerializer(serializers.ModelSerializer):
    class Meta:
        model = Profile
        fields = ['date_of_birth','phone_number','address']


class UserRegistrationSerializer(serializers.ModelSerializer):
    profile = ProfileSerializer(required=True)
    password = serializers.CharField(write_only=True)
    password2 = serializers.CharField(write_only=True, label='Confirm Password')

    class Meta:
        model = User
        fields = ['username', 'first_name', 'last_name', 'email', 'password', 'password2', 'profile']

    def create(self, validated_data):
        profile_data = validated_data.pop('profile')
        password = validated_data.pop('password')
        user = User.objects.create(**validated_data)
        user.set_password(password)
        user.save()
        Profile.objects.create(
            user=user,
            date_of_birth=profile_data.get('date_of_birth'),
            phone_number=profile_data.get('phone_number'),
            address=profile_data.get('address')
        )
        return user

Also you could use source attr at serialiser field:

class UserRegistrationSerializer(serializers.ModelSerializer):
    phone_number = serializers.CharField(source='profile.phone_number', required=False, max_length=30)
    date_of_birth = serializers.DateField(source='profile.date_of_birth', required=False)
    address = serializers.CharField(source='profile.address', required=False, max_length=250)
    password = serializers.CharField(write_only=True)
    password2 = serializers.CharField(write_only=True, label='Confirm Password')

    class Meta:
        model = User
        fields = ['username', 'first_name', 'last_name', 'email', 'phone_number', 'date_of_birth', 'address', 'password', 'password2']

    def create(self, validated_data):
        profile_data = validated_data.pop('profile', {})
        password = validated_data.pop('password')
        user = User.objects.create(**validated_data)
        user.set_password(password)
        user.save()
        Profile.objects.create(
            user=user,
            phone_number=profile_data.get('phone_number'),
            date_of_birth=profile_data.get('date_of_birth'),
            address=profile_data.get('address')
        )
        return user

This issue is due to how Django Rest Framework handles fields in serializers and models.

The Root Cause

You're using a ModelSerializer based on the User model, but you've added fields phone_number, date_of_birth, and address that belong to the Profile model, not the User model.

When DRF serializes or deserializes the User object, it expects that all fields in the serializer correspond to attributes on the User instance. Since address, phone_number and date_of_birth are fields on the Profile model, not the User model, DRF doesn't know how to access them.

In contrast, when you set write_only=True for the address field, it only allows the serializer to accept input for that field without rying to read it from the User object, wihch is why the error goes away. But this is only a partial solution.

Why only address causes an error

When you have required=False, DRF doesn't complain about missing attributes on theUser model because those fields are not required. This is why you get an error for only address. If you set it True, you'll get the same error.

Solution

You need to separate the concerns of serializing the User and Profile models. The best way to do this is to use a nested serializer for the Profile fields.

Here's the updated code:

serializers.py

Create a separate ProfileSerialzier and nest it inside your UserRegisterationSerializer:

class ProfileSerializer(serializers.ModelSerializer):
    class Meta:
        model = Profile
        fields = ['date_of_birth', 'phone_number', 'address']


class UserRegistrationSerializer(serializers.ModelSerializer):
    profile = ProfileSerializer()

    password = serializers.CharField(write_only=True)
    password2 = serializers.CharField(write_only=True, label='Confirm Password')

    class Meta:
        model = User
        fields = ['username', 'first_name', 'last_name', 'email', 'password', 'password2', 'profile']

    def create(self, validated_data):
        profile_data = validated_data.pop('profile', {})
        password = validated_data.pop('password', None)
        first_name = validated_data.pop('first_name', None)
        last_name = validated_data.pop('last_name', None)

        user = User(
            username=validated_data['username'],
            email=validated_data['email'],
            first_name=first_name,
            last_name=last_name
        )
        user.set_password(password)
        user.save()

        Profile.objects.create(
            user=user,
            date_of_birth=profile_data.get('date_of_birth', None),
            phone_number=profile_data.get('phone_number', None),
            address=profile_data.get('address', None),
        )

        return user

Sample request data

{
    "username": "testuser",
    "first_name": "Test",
    "last_name": "User",
    "email": "testuser@example.com",
    "password": "password123",
    "password2": "password123",
    "profile": {
        "date_of_birth": "1990-01-01",
        "phone_number": "1234567890",
        "address": "Test Street"
    }
}

I hope this will help you a little.