need some help with routes maybe

Im making a Cart for my site, so I make Orders Controller to manage
Orders

so inside I do

  def add_product
    @product = Product.find(params[:id])
    @order = Order.find_by_client_id(current_user.id)
    @order.products << @product
  end

on Product view/show I place
<%= link_to t(:add_to_cart), order_add_product_path(@product) %>

config/routes
  resources :orders do
    get :add_product
  end

But i kept getting

http://localhost:3000/orders/3/add_product
ActiveRecord::RecordNotFound in OrdersController#add_product
Couldn't find Product without an ID

model:
orders has_and_belongs_to_many :products
products has_and_belongs_to_many :orders