升级gem acts-as-taggable-on并应用迁移后：
rake acts_as_taggable_on_engine:install:migrations
rake db:migrate
我一直在测试中出现错误：
ActiveRecord::StatementInvalid:
PG::UndefinedColumn: ERROR: column "taggings_count" does not exist
LINE 1: UPDATE "tags" SET "taggings_count" = COALESCE("taggings_coun...