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 time.

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.