Historical Currency Exchange Rates with openexchangerates.org- Getting USD to other currencies conversion rates with openexchangerates api

exchange_rates.rb

require 'rubygems'
require 'json'
require 'active_support/core_ext'
require 'date'
require 'rest-client'
require 'fastercsv'

APP_ID = "<your_app_id>" 
# e.g. Get monthy currency (USD) exchange rates from 31st Jan 2008 to 31st Dec 2010
# http://openexchangerates.org/api/historical/2009-10-10.json?&app_id=<your_app_id>
start_date = Date.parse("2007-21-31")
 
FasterCSV.open("ExchangeRates.csv","w") do |csv|
 csv << ["ISO","USD_MID","DATE"]
 (1..36).each do |month|
   historical_date = (start_date >> month).end_of_month.strftime('%Y-%m-%d')
   puts "Loading USD conversion rates for #{historical_date}"
   json_response = JSON.parse(RestClient.get("http://openexchangerates.org/api/historical/#{historical_date}.json?&app_id=#{APP_ID}").body)
   json_response["rates"].each do |iso,usd_rate|
   	 csv << [iso,usd_rate,historical_date]
   end	 
   puts "Data loaded for #{historical_date}"
 end  
end

# Json response for a particular date is like :-
 # {
 #  "base": "USD",
 #  "rates": {
 #    "AED": 3.67285,
 #    "AFN": 48.930045,
 #    "ALL": 92.773005,
 #    "AMD": 385.739584,
 #    "ANG": 1.7825,
 #    "AOA": 75,
 #    "ARS": 3.832501,
 #    "AWG": 1.79025,
 #    "AZN": 0.803298,
 #    "BAM": 1.330168,
 #    "BBD": 2,
 #    "BDT": 68.732528,
 #    .
 #    .
 #   }
 # }