I haven't been using user_id for a while (we use LDAP), but now v. 2.11 complains about missing user_id in the class list file:
argument 1 contains empty 'user_id' field at /app/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/UserList2.pm line 1465.
at /app/webwork/webwork2/lib/WeBWorK/ContentGenerator/Instructor/UserList2.pm line 1465.
It looks like all students are imported correctly from the class list file despite the error message.
Similarly, when I add students manually in the Classlist Editor the "Student ID" label has a red star after it suggesting that it is mandatory, although students seem to be created if I leave empty that field.
Is empty user_id going to cause any problem?
The user_id field in a class list (.lst) file corresponds to the "Login Name" field in the Classlist Editor. As far as I know, this can not be blank. In fact, I would expect everything to break if it were blank because of the way the database and code are structured.
When you import students from a file with blank user_id, what shows up in the "Login Name" column in the Classlist Editor?
I think the reason that Student ID is labelled as mandatory is because this is what is used as the default password for the new user. If you leave it blank, it's possible that no password would be stored for the new user. This wouldn't be a problem in your case because you are not checking passwords against the WeBWorK database.