Controllers Should Convert Parameters to Richer Types

As the invokers of business logic, controllers are responsible for converting parameters into properly typed objects:

def show
  @widget = Widget.find(params[:id])
end

This code takes a string containing an identifier that we assume identifies a widget and looks it up in the database, passing the actual widget to the view. Because HTTP is a text-based protocol, and because Rails provides us only hashes of strings as an API into it, controllers are in the unique position to insulate the rest of the codebase from this ...