相关疑难解决方法(0)

我已经安装了mongo和bson_ext,现在我创建了一个带有以下内容的.rb文件::

require 'rubygems'
require 'mongo'

db = Mongo::Connection.new.db("mydb")
db = Mongo::Connection.new("localhost").db("mydb")
db = Mongo::Connection.new("localhost", 27017).db("mydb")

但是,我在运行代码时遇到以下错误

yuzaki@ubuntu:~$ ruby firstruby.rb
/home/ryuzaki/.rvm/gems/ruby-1.9.2-p136/gems/mongo-1.2.0/lib/mongo/connection.rb:451:in `connect': Failed to connect to a master node at localhost:27017 (Mongo::ConnectionFailure)
    from /home/ryuzaki/.rvm/gems/ruby-1.9.2-p136/gems/mongo-1.2.0/lib/mongo/connection.rb:554:in `setup'
    from /home/ryuzaki/.rvm/gems/ruby-1.9.2-p136/gems/mongo-1.2.0/lib/mongo/connection.rb:98:in `initialize'
    from firstruby.rb:4:in `new'
    from firstruby.rb:4:in `<main>'

请帮忙!