Rubyonrails ActiveRecord：：SubclassNotFound：单表继承机制定位子类失败

我是Rails的新手

class Category < ActiveRecord::Base
 attr_accessible :type

 has_many :products
end

class Product < ActiveRecord::Base
 attr_accessible :category_id, :color, :price, :title

 belongs_to :category
end

我的schema.rb分如下:-

ActiveRecord::Schema.define(:version => 20130725220046) do

create_table "categories", :force => true do |t|
    t.string   "type"
    t.datetime "created_at", :null => false
    t.datetime "updated_at", :null => false
  end

  create_table "products", :force => true do |t|
    t.integer  "category_id"
    t.decimal  "price",       :precision => 10, :scale => 0
    t.string   "title"
    t.string   "color"
    t.datetime "created_at",                                 :null => false
    t.datetime "updated_at",                                 :null => false
  end

end

在Rails控制台中，我使用Product.create命令创建了两个产品

[#<Product id: 1, category_id: 1, price: 500, title: "shirt", color: "blue", `created_at: "2013-07-25 22:04:54", updated_at: "2013-07-25 22:04:54">, #<Product id: 2, category_id: 1, price: 600, title: "tees", color: "black", created_at: "2013-07-25 22:05:17", updated_at: "2013-07-25 22:05:17">]`  

并在控制台中使用Category.create命令创建了两个类别

<Category id: 1, type: "clothing", created_at: "2013-07-25 22:03:54", updated_at: "2013-07-25 22:03:54"><Category id: 2, type: "footwear", created_at: "2013-07-25 22:04:02", updated_at: "2013-07-25 22:04:02">  

现在，Product.all可以，但Category.all可以

ActiveRecord::SubclassNotFound:单表继承机制无法定位子类:"clothing".引发此错误的原因是"type"列是为在继承情况下存储类而保留的.如果不打算将此列用于存储继承类或覆盖类别，请重命名此列.使用另一列获取该信息.

里面怎么了？我想在类别和产品之间建立一种关系，比如