Variables in Model Classes

There is no hard and fast rule, as far as CI is concerned.

My own practice is to use a Library for derived or run-time variables, with the Library calling the Model, which only contains variables that correspond to the database.

I did this for logical separation, but also convenience. It's very easy to save or load a Model that has a 1:1 between variables and db fields. From a programming standpoint I just call the Library from my Controller and it is up to the Library to know whether the variable comes from the Library or the Model.

I took inspiration for this methodology from the Ion-Auth library. You'll find some helpful magic methods in his Library files for doing this.