Django rest framework's ModelSerializer missing user field causing KeyError

Question

My ModelSerializer is getting a KeyError when I call .data on the serializer and I'm not sure how to properly fix this error.

Knox's AuthToken model looks like this

class AuthToken(models.Model):

    objects = AuthTokenManager()

    digest = models.CharField(
        max_length=CONSTANTS.DIGEST_LENGTH, primary_key=True)
    token_key = models.CharField(
        max_length=CONSTANTS.TOKEN_KEY_LENGTH, db_index=True)
    user = models.ForeignKey(User, null=False, blank=False,
                             related_name='auth_token_set', on_delete=models.CASCADE)
    created = models.DateTimeField(auto_now_add=True)
    expiry = models.DateTimeField(null=True, blank=True)

    def __str__(self):
        return '%s : %s' % (self.digest, self.user)

I created a Model serializer

class AuthTokenResponseSerializer(serializers.ModelSerializer):
    class Meta:
        model = AuthToken
        fields = "__all__"

However, when I create a token_serializer, I get this error

>> token_serializer.data
*** KeyError: "Got KeyError when attempting to get a value for field `user` on serializer `AuthTokenResponseSerializer`.\nThe serializer field might be named incorrectly and not match any attribute or key on the `dict` instance.\nOriginal exception text was: 'user'."

I think I am getting this error because when the token_serializer is created, it's missing a user key. I can see there is a user_id key but not user

(Pdb) token_serializer
AuthTokenResponseSerializer(context={'request': <rest_framework.request.Request: POST '/auth/token/'>}, data={'_state': <django.db.models.base.ModelState object>, 'digest': '6b82ac8776113db376d8938a641c3ebdea573b4ab49cb648c171fccc7f4a517e6450cebb21682e26f36562cf7b659f5089957ba49e99c096ebc9eaaa93ab0e53', 'token_key': '872832f8', 'user_id': 34, 'created': None, 'expiry': datetime.datetime(2024, 5, 11, 9, 17, 2, 540546, tzinfo=datetime.timezone.utc)}, partial=True):
    digest = CharField(max_length=128, validators=[<UniqueValidator(queryset=AuthToken.objects.all())>])
    token_key = CharField(max_length=8)
    created = DateTimeField(read_only=True)
    expiry = DateTimeField(allow_null=True, required=False)
    user = PrimaryKeyRelatedField(queryset=User.objects.all())

My User model

from django.db import models
from django.utils import timezone
from django.contrib.auth.models import (
    AbstractBaseUser,
    PermissionsMixin,
    BaseUserManager,
)
from django.utils.translation import gettext_lazy as _
from phonenumber_field.modelfields import PhoneNumberField

class User(AbstractBaseUser, PermissionsMixin):
    email = models.EmailField(
        verbose_name=_("email address"), unique=True, max_length=255
    )
    password = models.CharField(
        max_length=128, null=True
    ) 
    first_name = models.CharField(max_length=128, null=True)
    last_name = models.CharField(max_length=128, null=True)
    phone_number = PhoneNumberField(null=True, blank=True)
    is_staff = models.BooleanField(default=False)
    is_active = models.BooleanField(default=True)
    created_at = models.DateTimeField(default=timezone.now)
    updated_at = models.DateTimeField(default=timezone.now)