为什么我的has_many模型中只有一个有效

我紧随其后 http://railscasts.com/episodes/17-habtm-checkboxes-revised?view=asciicast 建立has_many直通关系的教程,当我试图访问一个模型的信息时,它可以工作,但不能访问另一个模型。

我可以通过访问产品模型中的类别信息 @product.category_ids 和 @product.categories ,但事实并非如此。我无法访问类别模型中的产品信息。使用 @category.product_ids 或 @category.products 给了我错误 NoMethodError: undefined method 'product_ids' for #<Category:0x007fa70d430e98>

产品.rb

class Product < ActiveRecord::Base
  attr_accessible  :category_ids

  has_many :categorizations
  has_many :categories, through: :categorizations
  accepts_nested_attributes_for :categorizations,  :allow_destroy => true
end

类别.rb

class Category < ActiveRecord::Base
  attr_accessible  :product_ids

  has_many :categorizations
  has_many :products, through: :categorizations
end

--编辑--

示意图.rb

ActiveRecord::Schema.define(:version => 20130926192205) do

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

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

  create_table "categorization", :force => true do |t|
    t.integer  "product_id"
    t.integer  "category_id"
    t.datetime "created_at",     :null => false
    t.datetime "updated_at",     :null => false
  end

  add_index "categorization", ["product_id", "category_id"], :name => "index_categorization_on_product_id_and_category_id", :unique => true
  add_index "categorization", ["product_id"], :name => "index_categorization_on_product_id"
  add_index "categorization", ["category_id"], :name => "index_categorization_on_category_id"

end