add user_user support to atlas collections

generic/models.py

@@ -470,12 +470,14 @@ class ExamCollectionGenericBase(models.Model):
             if c:
                 return c
 
-        if user_user is not None:
+        elif user_user is not None:
             c = CidUserExam.objects.filter(
                 content_type=content_type, object_id=self.pk, user_user=user_user
             ).first()
             if c:
                 return c
+        else:
+            raise ValueError("Invalid cid / cid_user / user_user")
 
         if start_time is None:
             start_time = timezone.now()
@@ -488,13 +490,15 @@ class ExamCollectionGenericBase(models.Model):
                 start_time=start_time,
             )
 
-        if user_user is not None:
+        elif user_user is not None:
             new = CidUserExam(
                 content_type=content_type,
                 object_id=self.pk,
                 user_user=user_user,
                 start_time=start_time,
             )
+        else:
+            raise ValueError("Invalid cid / cid_user / user_user")
 
         new.save()
         return new