simple question about code in a model class.

You'll hear more than one point of view on this issue, here's mine:

Storing business logic that is specific to the data a model represents
is what models are for. Do you find that there are complex things you
do with your models, over and over in many places? Factor out that
code and drop it onto the model, adding it as a method. Then you can
just ask the model for the answer rather than slogging through it each

I'd recommend taking a look at some actual source code to see how
models are used. Technoweenie's Beast is a good example... in ,
compare the Topic and User models to the Monitorship model to see the
range of possibilities.