Ruby on rails activerecord信誉系统不工作

我做了一个小项目参考快速启动，但当我访问信誉值时，出现了一个错误

回答：声誉：平均评级

有人帮我吗

假设我们想在问答网站中跟踪用户业力，其中用户业力是提问技能和回答技能的总和。提问技巧是用户问题的投票总数，回答技巧是用户答案平均评分的总和。这可以定义如下：

---

尝试从应答模型中删除：as=>：author，并在初始化中设置：of=>：user的has\u声誉：avg\u评级。

您应该发布更多代码。

NoMethodError: undefined method `to_sym' for nil:NilClass
from /Users/Juo/.rvm/gems/ruby-2.0.0-p195/bundler/gems/activerecord-reputation-system-01197ad78cac/lib/reputation_system/models/reputation.rb:189:in `get_target_type_for_sti'
from /Users/Juo/.rvm/gems/ruby-2.0.0-p195/bundler/gems/activerecord-reputation-system-01197ad78cac/lib/reputation_system/models/reputation.rb:198:in `set_target_type_for_sti'
from /Users/Juo/.rvm/gems/ruby-2.0.0-p195/gems/activesupport-4.0.0/lib/active_support/callbacks.rb:377:in `_run__3152603553061474011__validation__callbacks'`

class User < ActiveRecord::Base
  has_many :answers
  has_many :questions

  has_reputation :karma,
      :source => [
          { :reputation => :questioning_skill, :weight => 0.8 },
          { :reputation => :answering_skill }]

  has_reputation :questioning_skill,
      :source => { :reputation => :votes, :of => :questions }

  has_reputation :answering_skill,
      :source => { :reputation => :avg_rating, :of => :answers }
end

class Answer < ActiveRecord::Base
  belongs_to :user, :as => :author

  has_reputation :avg_rating,
      :source => :user,
      :aggregated_by => :average,
      :source_of => [{ :reputation => :answering_skill, :of => :author }]
end


class Question < ActiveRecord::Base
  belongs_to :user

  has_reputation :votes,
      :source => :user
end