update: use settings.AUTH_USER_MODEL

djangoldp_notification/factories.py

 import factory
+from django.conf import settings
+
 from .models import Notification
-from django.contrib.auth.models import User
 from django.db.models.signals import post_save
 
 @factory.django.mute_signals(post_save)
@@ -11,7 +12,7 @@ class NotificationFactory(factory.django.DjangoModelFactory):
     type = factory.Faker('text', max_nb_chars=50)
     summary = factory.Faker('paragraph', nb_sentences=3, variable_nb_sentences=True)
     author_user = factory.Faker('url')
-    user = factory.Iterator(User.objects.all())
+    user = factory.Iterator(settings.AUTH_USER_MODEL.objects.all())
     date = factory.Faker('past_datetime')
     read = factory.Faker('boolean')
     object = factory.Faker('url')
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