Mongoid 的 里面有 Regexp 的 类型

我定义如下

field :name_reg, :type => Regexp

运行却报错，请教一下，如何把 正则表达式存储到 mongodb 里面 ××××××××××××××× Routing Error

undefined method `instantiate' for Regexp:Class Try running rake routes for more information on available routes. ×××××××××××××××××

Below is a list of valid types for fields.

Array BigDecimal Boolean Date DateTime Float Hash Integer Moped::BSON::ObjectId Moped::BSON::Binary Range Regexp String Symbol Time TimeWithZone