[#45455707] カテゴリの追加
[#45455707] Add script for prepare categories to db/seeds.rb
[#45455707] Add reference for category and product
[#45455707] 商品登録フォームの変更
[#45455707] Can't mass-assign protected attributes: エラーの修正
[#45455707] Show product category on product pages
[#45455707] category_spec.rbの削除
Merge pull request #13 from merume/add_product_category

[#45455707] Add product category