class Purchase
  def initialize(cart_item_ids)
    @cart_item_ids = cart_item_ids
  end

  def has_cart_items?
    !@cart_item_ids.empty?
  end

  def paypal_url
    # ...
  end
end

class PurchasesController < ActionController::Base
  def create
    @purchase = Purchase.new(session[:cart_items])
    if @purchase.has_cart_items?
      redirect_to @purchase.paypal_url
    else
      flash[:notice] = "Your shopping cart is empty.  Please add a product and try again."
      redirect_to "/store"
    end
  end
end

map.resources :purchases