こんにちは、はじめまして。
最近rubyとrailsを集中して勉強し始めたsonoといいます。
どうぞよろしくおねがいします。

現在、「railsによるアジャイルウェブアプリケーション開発」本を
読みすすめながら、実際に動かして試しているのですが
85〜86ページあたりのカート作成の部分で
どうしてもエラーが出てしまいます。
本自身は日本語の第一版でした。

エラー内容は以下です。

NameError in StoreController#add_to_cart

undefined local variable or method `product' for LineItem:Class
RAILS_ROOT: script/../config/..

Application Trace | Framework Trace | Full Trace
/usr/local/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1129:in
`method_missing'
#{RAILS_ROOT}/app/models/line_item.rb:6:in `for_product'
#{RAILS_ROOT}/app/models/cart.rb:11:in `add_product'
app/controllers/store_controller.rb:10:in `add_to_cart'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/base.rb:910:in
`send'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/base.rb:910:in
`perform_action_without_filters'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/filters.rb:368:in
`perform_action_without_benchmark'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/benchmarking.rb:69:in
`perform_action_without_rescue'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/benchmarking.rb:69:in
`measure'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/benchmarking.rb:69:in
`perform_action_without_rescue'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/rescue.rb:82:in
`perform_action'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/base.rb:381:in
`send'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/base.rb:381:in
`process_without_filters'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/filters.rb:377:in
`process_without_session_management_support'
/usr/local/lib/ruby/gems/1.8/gems/actionpack-1.12.3/lib/action_controller/session_management.rb:117:in
`process'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/dispatcher.rb:38:in
`dispatch'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/webrick_server.rb:115:in
`handle_dispatch'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/webrick_server.rb:81:in
`service'
/usr/local/lib/ruby/1.8/webrick/httpserver.rb:104:in `service'
/usr/local/lib/ruby/1.8/webrick/httpserver.rb:65:in `run'
/usr/local/lib/ruby/1.8/webrick/server.rb:155:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:144:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:144:in `start_thread'
/usr/local/lib/ruby/1.8/webrick/server.rb:94:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:89:in `each'
/usr/local/lib/ruby/1.8/webrick/server.rb:89:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:79:in `start'
/usr/local/lib/ruby/1.8/webrick/server.rb:79:in `start'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/webrick_server.rb:67:in
`dispatch'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/commands/servers/webrick.rb:59
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`gem_original_require'
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.3.1/lib/active_support/dependencies.rb:147:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/rails-1.1.4/lib/commands/server.rb:30
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`gem_original_require'
/usr/local/lib/ruby/site_ruby/1.8/rubygems/custom_require.rb:27:in
`require'
/usr/local/lib/ruby/gems/1.8/gems/activesupport-1.3.1/lib/active_support/dependencies.rb:147:in
`require'
script/server:3
Request

Parameters: {"id"=>"2"}

Show session dump

--- 
:cart: !ruby/object:Cart 
  items: []
  tital_price: 0.0
flash: !ruby/hash:ActionController::Flash::FlashHash {}
Response

Headers: {"cookie"=>[], "Cache-Control"=>"no-cache"}


そこで、英語版MLのほうで
http://lists.rubyonrails.org/pipermail/rails/2006-January/012512.html
ここの記事は見つけたのですが、
Cartクラスがおかしいのでは、→複数形の誤植があった？
というようなところまでは読み取ったんですが、
英語力が乏しいのと、英語版の訂正リストを見ようにも日本語版で
該当ページを見つけられませんでした。

この件に付きまして、なにかご存知の方がございましたら
ヒントをいただけませんでしょうか。

よろしくおねがいいたします。

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