先执行：ruby script/generatemigration add_change_flag_to_users change_flag:Boolean

生成时间序列文件：db/migrate/20131016034104_add_changeflag_to_users.rb

内容：

class AddchangeflagToUsers< ActiveRecord::Migration

def self.up

  add_column :users, :change_flag, :boolean

end

def self.down

  remove_column :users, :change_flag

end

end

手动写入 default =>true  设置change_flag默认值1

class AddchangeflagToUsers< ActiveRecord::Migration

def self.up

  add_column :users, :change_flag, :boolean, default=> true

end

def self.down

  remove_column :users, :change_flag

end

end

执行 rake db:migrate

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