[onion2k]

Think of the value as a user preference along the lines of "Do you want to see the product tour? [Yes|No]". How would you store any similar user-set preference?

In Usable Requirements (http://www.usablehq.com) we use an untracked JSON object as part of the user document for preferences (this is in MongoDB). This means we don't have to keep a schema up to date - the front end can essentially put anything in that object (subject to some validation obviously). Consequently the front end code has to be tremendously defensive as values may or may not be there, but that's a good thing anyway.