This works:

class Library
  include Mongoid::Document
  identity :type => Integer
end
class Book
  include Mongoid::Document  
  belongs_to :library
end

Library.delete_all
Book.delete_all

library = Library.create!(:id => 1)
Book.create!(:library_id => 1)
p Book.first.library

prints

#<Library _id: 1, _type: nil>

But if you change it to this:

Book.create!(:library_id => "1")

It fails:

nil

Why does this matter? Oftentimes in Rails you use params to create/update Mongoid objects. Params are always strings. So if you change your identity type from BSON::ObjectId to Integer, things suddenly stop working. You can workaround the bug by using callbacks to set the correct type, but that seems like a hack.