Part of a rails app

I am working on a small project and I am stuck. I want you to create a model Product with name: string and qty: integer . On products/show page I need one button named ''USE" . If you click USE button to send you to another view, let say "Use Product" where we have one input (in this input we can put a quantity not bigger than @[login to view URL]) and another button 'USE Product". When you click USE Product button the rails DB should be update with new quantity ( @[login to view URL] - value of input ). Thank you.