使用update_attribute运行Rails 3迁移时出现只读错误

Question

我正在尝试添加一个counter_cache,这个Rails 3迁移给我一个我似乎无法解决的错误.

迁移是

class AddItemsCountToStore < ActiveRecord::Migration
    def self.up
        add_column :stores, :items_count, :integer, :default => 0

        Store.reset_column_information

        Store.all.each do |store|
            store.update_attribute :items_count, store.items.count
        end
    end

    def self.down
        remove_column :stores, :items_count
    end
end

而错误是:

== AddItemsCountToStore:迁移========================================= - add_column( :stores,:items_count,:integer,{:default => 0}) - > 0.0680s rake aborted!发生错误,此以及所有后续迁移都已取消:

items_count被标记为只读C:/Ruby192/lib/ruby/gems/1.9.1/gems/activerecord-3.0.0/lib/active_record/persistence.rb:115:在`update_attribute"

有任何想法吗？