ruby on rails 3 - how can i redirect to specific link out of my app when devise before_filter :authenticate_user works -

here categories controller code

   class categoriescontroller < applicationcontroller       before_filter :authenticate_user! ,:except => :index       def index        #some code      end       def show        #some code      end     end   

clearly can see categories#index page publicly view-able on categories listed. when click on category name without sign in, he/she redirect 'localhost:3000/users/sign_in'.

but want user redirect url "www.facebook.com" whenever devise before_filter :authenticate_user! work.

how can achieve this?