Intercepting form submission

Where (what function name must I insert in my controller, I assume)
can I obtain the selected value from a field in a form, to process and
change it before it gets written to the model's table in the db?
Thanks, Janna B.

Well, you *could* choose the ugly path and just place that code in the
controller create and/or update methods, after your model is
reconstituted from the params.

Or choose a better path and place that code in a method in the model,
and define a before_save in the model that invokes the method
(before_save is called for either create or update).