NameError: 未初始化的常量 <Model>:<column_name>

Question

我在我的 Rails 设置中遇到了一个奇怪的问题，这让我百思不得其解。 我正在尝试在用户和书籍之间创建以下关系

class Book < ActiveRecord::Base   
  has_many :follows, :dependent => :destroy
  belongs_to :user
end

class Follow < ActiveRecord::Base
  attr_accessible :user_id,
                  :book_id,
                  :updated_at
  belongs_to :user
  belongs_to :book_id
  validates :user_id, presence: true
  validates :book_id, presence: true
end

class User < ActiveRecord::Base   
  has_many :books, :dependent => :destroy
  has_many :follows, :dependent => :destroy
end

直观地说，一个用户可以拥有许多书籍，并且可以关注其他书籍（来自其他用户） 我希望我的关注模型跟踪这种关系

这是我的数据库的设置方式 (PostgreSQL)

# \d follows
                                     Table "public.follows"
   Column   |            Type             |                      Modifiers                       
------------+-----------------------------+------------------------------------------------------
 id         | integer                     | not null default nextval('follows_id_seq'::regclass)
 user_id    | integer                     | 
 book_id    | integer                     | 
 created_at | timestamp without time zone | 
 updated_at | timestamp without time zone | 
Indexes:
    "follows_pkey" PRIMARY KEY, btree (id)
    "index_follows_on_book_id_and_user_id" UNIQUE, btree (book_id, user_id)
    "index_follows_on_book_id" btree (book_id)
    "index_follows_on_user_id" btree (user_id)

最后，当我打开 Rails 控制台并尝试创建一个 Follow 实例并分配一个 book_id 值时，它会在标题中吐出消息：

导轨 c 加载开发环境（Rails 4.0.0.rc1） 2.0.0p247 :001 > f = Follow.new => # 2.0.0p247:004 > f.book_id = 48 NameError: 未初始化的常量 Follow::BookId 来自/Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/inheritance.rb:125:incompute_type' from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/reflection.rb:178:inklass' 来自/Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/associations/association.rb:207:in raise_on_type_mismatch!' from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/associations/belongs_to_association.rb:11:inreplace' 来自/Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/associations/singular_association.rb:17:in writer' from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/associations/builder/association.rb:78:inbook_id=' 来自 (irb):4 来自 /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0.rc1/lib/rails/commands/console.rb:90:in start' from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0.rc1/lib/rails/commands/console.rb:9:instart' 来自/Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/railties-4.0.0.rc1/lib/rails/commands.rb:66:in <top (required)>' from bin/rails:4:inrequire' 从 bin/rails:4:in `'

如果有人能说出发生了什么并给出一些提示，那就太棒了。 提前致谢。

Answer 1

变化：

belongs_to :book_id

到

belongs_to :book

另外，您真的希望将 book_id 和 user_id 标记为可访问吗？这可能允许使用精心设计的帖子请求更新这些字段。