gistfile1.rb

# invoices_controller.rb 
  
  helper_method :available_filters

  def index
     @invoices = Invoice.send(filter_scope)
  end

  private 
    def available_filters
      ["unpaid", "paid", "whatever"]
    end

    def filter_scope
      filtered? ? params[:filter].to_sym : :all
    end

    def filtered?
      available_filters.include? params[:filter]
    end 
    
# in the view
<ul>
  <li><%= link_to "all", invoices_path %></li>
  <% available_filters.each do |filter| %>
    <li><%= link_to filter, invoices_path(:filter => filter) %></li>
  <% end %>
</ul>