The problem is when I want to format the output of attributes in that
method. For example I want to format the attribute price to currency.
There is a nice helper method called number_to_currency. How can I use
this in my to_s method in my model?
Helpers are just modules.. I'd say you can include them as you would with any other. Only problem would be if any of your helper methods relies on the classes that are under Controllers' hierarchy tree. If not, everything should be fine.