foreignkey update

models.py

from django.contrib.auth.models import User
from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType
from django.db import models

class Book(models.Model):
    author = models.ForeignKey(User, on_delete=models.CASCADE)

class Comment(models.Model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id = models.PositiveIntegerField()
    author = GenericForeignKey()