line_items_controller.rb def create @cart = current_cart product = Product.find(params[:product_id]) @line_item = @cart.line_items.build(product_id: product.id)

respond_to do |format| if @line_item.save format.html { redirect_to @line_item.cart, notice: 'Line item was successfully created.' } format.json { render json: @line_item, status: :created, location: @line_item } else format.html { render action: "new" } format.json { render json: @line_item.errors, status: :unprocessable_entity } end end end

carts_controller.rb

def show @cart = Cart.find(params[:id])

respond_to do |format| format.html # show.html.erb format.json { render json: @cart } end end app/views/carts/show.html.erb

<%= notice %>

Your programatic Cart

<% @cart.line_items.each do |item| %>
• <%= item.product.title %>
<% end %>

<%= link_to 'Edit', edit_cart_path(@cart) %> | <%= link_to 'Back', carts_path %>

<%= button_to 'Add to Cart', line_items_path(product_id: product) %>

问题： button_to 到 line_items#create，然后 redirect_to "carts#show" 最后到 show.html.erb

出错：undefined method `title' for nil:NilClass