foursquare_oauth_sample.rb

require 'sinatra'
require 'open-uri'
require 'json'

OAUTH_URL = 'https://foursquare.com/oauth2'
BASE_URL = 'https://api.foursquare.com/v2'
REDIRECT_URI = 'http://localhost:9292/auth'
CLIENT_ID = 'YOUR_CLIENT_ID'
CLIENT_SECRET = 'YOUR_CLIENT_SECRET'

get '/' do
  redirect %(#{OAUTH_URL}/authenticate?client_id=#{CLIENT_ID}&response_type=code&redirect_uri=#{REDIRECT_URI})
end

get '/auth' do
  res = open(%(#{OAUTH_URL}/access_token?client_id=#{CLIENT_ID}&client_secret=#{CLIENT_SECRET}&grant_type=authorization_code&redirect_uri=#{REDIRECT_URI}/auth&code=#{params['code']}))
  json = JSON.parse(res.string)
  $token = json['access_token']
  redirect '/checkins'
end

get '/checkins' do
  res = open(%(#{BASE_URL}/users/self/checkins?oauth_token=#{$token}))
  json = JSON.parse(res.read)
  json.inspect
end