...

#routes.rb

   get '/users/:user_id/account/:id' => 'foo#bar', as: 'foo_bar'

...

...

#foo_controller.rb

def bar
  flash[:warning] = "You have found Bar!."
  #return redirect_to x <--- still nothing!
end

...

<%= link_to 'Get', foo_bar_path, class: "input-group-button button", method: :get %>