improve case user answer management (and fix small bug)

generic/models.py

@@ -613,6 +613,8 @@ class ExamOrCollectionGenericBase(models.Model, AuthorMixin):
         default=False,
     )
 
+    exam_open_access = models.BooleanField(default=False, help_text="Set to true if you want any registered user to be able to take the exam.")
+
     class Meta:
         abstract = True
 
@@ -718,6 +720,8 @@ class ExamOrCollectionGenericBase(models.Model, AuthorMixin):
                     return True
 
         if cid is None and user_id is None:
+            if user.is_anonymous:
+                return False
             if user is not None:
                 user_id = user.id
             else:
@@ -869,8 +873,6 @@ class ExamBase(ExamOrCollectionGenericBase):
         default=False,
     )
 
-    exam_open_access = models.BooleanField(default=False, help_text="Set to true if you want any registered user to be able to take the exam.")
-
     # randomise_question_order = models.BooleanField(
     #    help_text="If an exam should randomise the order of questions in RTS.",
     # )