假设如下:
irb> x
irb> => {"_id"=> 123456, "welcome"=>"Hi!", "welcome2" => "Enjoy your stay!"}
irb> coll.class
irb> => Mongo::Collection
如何使用原始的mongo-ruby-driver通过重写方法和原子更新方法来更新x对应的文档?(见http://api.mongodb.org/ruby/current/file.TUTORIAL.html#Updating_a_Document)