Django model foreign key to whichever model calls it

I am getting back into Django after a few years, and am running into the following problem. I am making a system where there are 2 models; a survey, and an update. I want to make a notification model that would automatically have an object added when I add a survey object or update object, and the notification object would have a foreign key to the model object which caused it to be added.

However I am running into a brick wall figuring out how I would do this, to have a model with a foreign key which can be to one of two models, which would be automatically set to the model object which creates it. Any help with this would be appreciated.

I am trying to make a model that looks something like this (psuedocode):

class notification(models.model):
     source = models.ForeignKey(to model that created it) #this is what I need help with
     start_date = models.DateTimeField(inherited from model that created it)
     end_date = models.DateTimeField(inherited from model that created it)

Also, just to add some context to the question and in case I am looking at this from the wrong angle, I am wanting to do this because both surveys and updates will be displayed on the same page, so my plan is to query the notification model, and then have the view do something like this:

from .models import notification

notifications = notification.objects.filter(start_date__lte=now, end_date__gte=now).order_by('-start_date')

for notification in notifications:
     if notification.__class__.__name__ == "survey_question":
          survey = notification.survey_question.all()
          question = survey.question()
     elif notification.__class__.__name__ == "update":
          update = notification.update.all()
          update = update.update()

I am also doing this instead of combining the 2 queries and then sorting them by date as I want to have notifications for each specific user anyways, so my plan is (down the road) to have a notification created for each user.

Here are my models (that I reference in the question):

from django.db import models
from datetime import timedelta
from django.utils import timezone

def tmrw():
    return timezone.now() + timedelta(days=1)

class update(models.Model):
    update = models.TextField()
    start_date = models.DateTimeField(default=timezone.now, null=True, blank=True)
    end_date = models.DateTimeField(default=tmrw, null=True, blank=True)

    class Meta:
        verbose_name = 'Update'
        verbose_name_plural = f'{verbose_name}s'

class survey_question(models.Model):
    question = models.TextField()
    start_date = models.DateTimeField(default=timezone.now, null=True, blank=True)
    end_date = models.DateTimeField(default=tmrw, null=True, blank=True)

    class Meta:
        verbose_name = 'Survey'
        verbose_name_plural = f'{verbose_name}s'

class SurveyQuestion(models.Model):    # Note use of PascalCase for class name

    SURVEY = "SV"
    UPDATE = "UP"

    SURVEY_TYPE_CHOICES = {
        SURVEY: "Survey",
        UPDATE: "Update",
    }
    survey_type = models.CharField(
        max_length=2,
        choices=SURVEY_TYPE_CHOICES,
        default=SURVEY,
    )
    question = models.TextField()
    start_date = models.DateTimeField(default=timezone.now, null=True, blank=True)
    end_date = models.DateTimeField(default=tmrw, null=True, blank=True)

from django.contrib.contenttypes.fields import GenericForeignKey
from django.contrib.contenttypes.models import ContentType

class notification(models.model):
    content_type = models.ForeignKey(ContentType, on_delete=models.CASCADE)
    object_id    = models.PositiveIntegerField()
    source       = GenericForeignKey("content_type", "object_id")