山海经

class AlterUsertype < ActiveRecord::Migration
 def self.up
   execute "update usertypes set position = position + 1 where position > 4"
   us = Usertype.create(:name => 'virtual_school')
   # 要单独设置position属性，是因为Usertype模型中使用了acts_as_list的特性
   us.position = 5
   us.save
 end
 
 def self.down
  Usertype.delete_all(['name = ?', 'virtual_school'])
  execute "update usertypes set position = position - 1 where position > 4"
  # 更新usertypes表的auto_increment属性
  execute "alter table usertypes auto_increment = 12"
 end
end