ruby – Mongoid如何知道字符串值和符号值之间的区别？

> x = User.first # or any persisted Mongoid::Document
=> #<User _id: 52014532a6356d1ac9000001,...>
> x.set :foo,:bar
=> :bar
> x.set :foo2,'bar'
=> "bar"

请注意,“foo”和“foo2”未在Ruby中声明.

那么,在MongoDB shell中：

> db.users.findOne({_id: ObjectId('52014532a6356d1ac9000001')})    
{
  "_id" : ObjectId("52014532a6356d1ac9000001"),"foo" : "bar","foo2" : "bar",...
}

但现在,回到Ruby：

> x = User.find x.id; nil # to clear out any possibility of metadata on the instance
=> nil
> [x.read_attribute(:foo),x.read_attribute(:foo2)]
=> [:bar,"bar"]

怎么知道的？