freebase_api.rb

require 'net/https'
require 'uri'
require 'cgi'

module FreebaseApi
  USER_AGENT = 'DbpediaLite/1'
  MQLREAD_URI = URI.parse('https://www.googleapis.com/freebase/v1/mqlread')
  RDF_BASE_URI = URI.parse('http://rdf.freebase.com/rdf/')
  GOOGLE_API_KEY = ENV['GOOGLE_API_KEY']
  HTTP_TIMEOUT = 2

  class Exception < Exception
  end

  class NotFound < FreebaseApi::Exception
  end

  def self.mqlread(query)
    uri = MQLREAD_URI.clone
    uri.query = 'query='+CGI::escape(JSON.dump(query))

    unless GOOGLE_API_KEY.nil?
      uri.query += "&key=#{GOOGLE_API_KEY}"
    end

    http = Net::HTTP.new(uri.host, uri.port)
    http.open_timeout = HTTP_TIMEOUT
    http.read_timeout = HTTP_TIMEOUT

    if uri.scheme == 'https'
      http.use_ssl = true
      http.verify_mode = OpenSSL::SSL::VERIFY_NONE
    end

    res = http.get(uri.request_uri, {'User-Agent' => USER_AGENT})

    # Throw an exception if HTTP request was unsuccessful
    res.value

    # Throw and exception if the JSON response was unsuccessful
    data = JSON.parse(res.body)
    if data.has_key?('error')
      raise FreebaseApi::Exception.new("Freebase query failed: #{data['error']['message']}")
    end

    if data['result'].nil?
      raise FreebaseApi::NotFound.new("Freebase query returned no results")
    end

    data['result']
  end

  def self.lookup_wikipedia_pageid(pageid, language='en')
    mqlread({
      'key' => {
        'namespace' => "/wikipedia/#{language}_id",
        'value' => pageid.to_s,
        'limit' => 0
       },
       'id' => nil,
       'name' => nil,
       'mid' => nil,
       'guid' => nil,
       'limit' => 1
    })
  end

  def self.lookup_by_id(identifier, language='en')
    mqlread({
      'id' => identifier,
      'key' => {
        'namespace' => "/wikipedia/#{language}_id",
        'value' => nil,
        'limit' => 1
      },
      'name' => nil,
      'mid' => nil,
      'guid' => nil,
      'limit' => 1
    })
  end

end