MacPortsでインストールしたTwitter GemでMultiJson::DecodeError

MacPorts-1.9.2を使ってインストールしたrubygemsでTwitter Gemを入れたところ少しハマったので残しておく。
MacPortsはパッケージインストーラで入れただけ。
その後rubygemsをインストール。

$ sudo port install rb-rubygems

ここで

$ which gem ruby

すると両方/opt/localの下を指しているが、

$ gem --version
$ ruby --version

の出力を見るとどうもMac標準の/usr/binの下が呼ばれていそうなので、シェルを一回再起動する。
次にTwitter Gemsをインストール。

$ sudo gem install twitter

動作を確認するため、サンプルを書く。

require 'rubygems'
require 'twitter'

p Twitter.user('junji_furuya').location

これを実行したところ、MultiJson::DecodeErrorになった。

/opt/local/lib/ruby/gems/1.8/gems/multi_json-0.0.5/lib/multi_json.rb:66:in `decode': #<RuntimeError: Did not recognize your engine specification. Please specify either a symbol or a class.> (MultiJson::DecodeError)
	from /opt/local/lib/ruby/gems/1.8/gems/faraday_middleware-0.3.2/lib/faraday/parse_json.rb:22:in `register_on_complete'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:47:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:47:in `finish'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:47:in `each'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:47:in `finish'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/builder.rb:19:in `inner_app'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:17:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/response.rb:17:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/adapter/net_http.rb:57:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/faraday/multipart.rb:16:in `call'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/request.rb:85:in `run'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/request.rb:27:in `run'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/connection.rb:177:in `run_request'
	from /opt/local/lib/ruby/gems/1.8/gems/faraday-0.5.7/lib/faraday/connection.rb:66:in `get'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter/request.rb:28:in `send'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter/request.rb:28:in `request'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter/request.rb:6:in `get'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter/client/user.rb:23:in `user'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter.rb:21:in `send'
	from /opt/local/lib/ruby/gems/1.8/gems/twitter-1.1.2/lib/twitter.rb:21:in `method_missing'
	from hello_world.rb:5

どうも何か設定が足りない様子。
ググってもよく分からなかったので、/opt/local/lib/ruby/gems/1.8/gems/multi_json-0.0.5/lib/multi_json.rbの中身を読と、12〜36行目付近に次のような記述があった。

  REQUIREMENT_MAP = [
    ["yajl", :yajl],
    ["json", :json_gem],
    ["active_support", :active_support],
    ["json/pure", :json_pure]
  ]

  # The default engine based on what you currently
  # have loaded and installed. First checks to see
  # if any engines are already loaded, then checks
  # to see which are installed if none are loaded.
  def default_engine
    return :yajl if defined?(::Yajl)
    return :json_gem if defined?(::JSON)
    return :active_support if defined?(::ActiveSupport::JSON)

    REQUIREMENT_MAP.each do |(library, engine)|
      begin
        require library
        return engine
      rescue LoadError
        next
      end
    end
  end

どうもREQUIREMENT_MAPの各要素の１番目の要素の文字列を順番にrequireしている様子。
要はJSONのエンジンがインストールされていないのが原因らしい。
そこでyajlをrubygemsからインストール。

$ gems search --remote yajl
benofsky-yajl-ruby (0.7.7)
filipegiusti-yajl-ruby (0.6.4)
sprout-yajl-library (0.0.1)
yajl-ruby (0.8.1)
$ sudo gems install yajl-ruby

これで、サンプルを動かすことができた。

$ ruby -Ku location.rb
"東京都北区"