使用Ruby语言的MongoMapper问题

我安装了MongoMapper gem,版本是0.8.2。我打开environment.rb文件并对其进行编辑以使用mongomapper,如下所示:

# Be sure to restart your server when you modify this file

# Specifies gem version of Rails to use when vendor/rails is not present
RAILS_GEM_VERSION = '2.3.8' unless defined? RAILS_GEM_VERSION

# Bootstrap the Rails environment, frameworks, and default configuration
require File.join(File.dirname(__FILE__), 'boot')

Rails::Initializer.run do |config|

  config.gem 'mongomapper', :version => '>= 0.2.1'
  config.frameworks -= [:active_record]


  config.time_zone = 'UTC'

end

MongoMapper.database = "blog-#{Rails.env}"

当我使用Ruby脚本/服务器启动服务器时,我得到以下异常:

Mohammad-Azams-MacBook-Pro:blog azamsharp$ ruby script/server
=> Booting Mongrel
=> Rails 2.3.8 application starting on http://0.0.0.0:3000
Missing these required gems:
  mongomapper  >= 0.2.1

You're running:
  ruby 1.8.7.174 at /usr/local/bin/ruby
  rubygems 1.3.7 at /Users/azamsharp/.gem/ruby/1.8, /usr/local/lib/ruby/gems/1.8