为什么我得到这个错误，如果我试图使计数器缓存

Question

我使用Rails 3.2.6。当我尝试使计数器缓存时，不知何故我得到了这个错误。我该如何解决这个问题?我在这个应用上做了同样的事情，但不是在这个模型上。
我的代码或关联有什么问题?

命令 bundle exec rake db:migrate

日志

==  AddCommunityTopicsCountToCommunity: migrating =============================
-- add_column(:communities, :community_topics_count, :integer, {:default=>0})
   -> 0.0635s
rake aborted!
An error has occurred, all later migrations canceled:
community_topics_count is marked as readonly

模型/community.rb

...
has_many :community_topics
...

模型/community_topic.rb

...
belongs_to :community, counter_cache: true
...

迁移文件

class AddCommunityTopicsCountToCommunity < ActiveRecord::Migration
  def up
    add_column :communities, :community_topics_count, :integer, :default => 0
    Community.reset_column_information
    Community.all.each do |p|
      p.update_attribute :community_topics_count, p.community_topics.length
    end
  end
  def down
    remove_column :communities, :community_topics_count
  end
end

Answer 1

class AddCommunityTopicsCountToCommunity < ActiveRecord::Migration
  def up
    add_column :communities, :community_topics_count, :integer, :default => 0
    Community.reset_column_information
    Community.all.each do |c|
      Community.reset_counters(c.id, :community_topics)
    end
  end
end

Answer 2

当你添加一个conter_cache，它是不可能更新它使用Rails设置为只读默认值

您可以通过将updated_attribute替换为update_column来绕过此操作，这将跳过任何验证或回调。

Answer 3

conter_cache有自己的处理方法，详见文档

你可以用

Community.all.each do |p|
  Community.reset_counters(p.id, :community_topics)
end