Rateable不能为空（mongoid-letrate，Mongoid）

Question

当我尝试创建新User(devise)时，我收到此错误：

Rateable can't be blank(mongoid-letsrate,Mongoid)
Mongoid::Errors::Validations:
   message:
     Validation of User failed.
   summary:
     The following errors were found: Rateable can't be blank
   resolution:
     Try persisting the document with valid data or remove the validations.

我的模特用户：

# User
class User
  include Mongoid::Document
  include Mongoid::Letsrate
   letsrate_rater
end

我的模特书：

# Book
class Book
  include Mongoid::Document
  include Mongoid::Timestamps
  include Mongoid::Letsrate
  letsrate_rateable
end