Django + DRF, Using an email address as a username

email_username.py

class User(AbstractUser):
    pass


class UserSerializer(serializers.ModelSerializer):
    username = serializers.EmailField()
    
    class Meta:
        model = User
    
    def validate_username(self, attrs, source):
        """
        Customize the error message for a duplicate username.
        Use "email" instead of "username"
        """
        if self.object is None:
            username = attrs[source]
            if User._default_manager.filter(username=username).exists():
                raise serializers.ValidationError("That email is already being used.", code='duplicate_username')
        return attrs
  

class UserViewSet(viewsets.ModelViewSet):
    model = User
    serializer_class = UserSerializer
    
    def pre_save(self, obj):
        obj.email = obj.username