Using values from a dropdown to populate a table

Hello,

What im trying to do is straightforward, but i cant seem to figure out how to go about doing it.

Referring to the images, i am trying to add a class to a user. A very simple method of choosing from a dropdown, click add class and presto its linked to the user. I would also like to make sure that the class is not added twice.

However i have no idea how i can just fill up that dropbox without it being a part of the table, just referencing from another table (Classes.class Name). After which when its linked, the linkage would be registered in the "Classes Joined" table.. linking the user id to the class id.

I got a minor idea to use a calculation field entry to temporary store the data.. or something like that.. or am i doing it all wrong?

I have created the rship between the tables, correctly, i think.. and also created the valuelist to load values from classes.. but i dont seem to be going anywhere... seems simple.. yet difficult.

Further trying i am able to load the values to the dropbox as can be seen in the picture, but because i use a calculation field i get the error "the action cannot be performed because this field is not modifiable"

Changing the field in the database to text would be ok, but i dont even intend to store that value.. just as a temporary just for the sake of selection to enter in to the database. Also what should the script to be used for the add Class so that it can just add it to the table?

Please help! THanks!