NameError: uninitialized constant <Model>:<column_name>

Question

I've encountered a strange problem in my Rails setup, that is puzzling me to no end. I'm trying to create a following relation between Users and Books

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

Intuitively, a User can have many Books, and can follow other Books (from other Users) I wanted my Follow model to track that relationship

Here is how my database is setup (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)

Finally, when I turn on Rails Console and try to create a Follow instance, and assign a book_id value, it spits out the message in the title:

rails c Loading development environment (Rails 4.0.0.rc1) 2.0.0p247 :001 > f = Follow.new => # 2.0.0p247 :004 > f.book_id = 48 NameError: uninitialized constant Follow::BookId from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/inheritance.rb:125:in compute_type' from /Users/minghuazhao/.rvm/gems/ruby-2.0.0-p247/gems/activerecord-4.0.0.rc1/lib/active_record/reflection.rb:178:inklass' from /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' from /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=' from (irb):4 from /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' from /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' from bin/rails:4:in `'

If someone can tell what's going on and give a few hints, that would be awesome. Thanks in advance.

Answer 1

Change:

belongs_to :book_id

to

belongs_to :book

Also, do you really want book_id and user_id to be marked as accessible? This may allow those fields to be updated with crafted post requests.