app.rb

require "rubygems"
require "sinatra"
require "oauth2"


get "/" do
  haml :index
end


get "/facebook/sign_in" do
  authorize_url = oauth_client.auth_code.authorize_url(redirect_uri: redirect_uri)

  redirect to(authorize_url)
end


get "/facebook/callback" do
  code         = params[:code]
  access_token = oauth_client.auth_code.get_token(code, redirect_uri: redirect_uri)

  redirect to('/')
end


def oauth_client
  api_key      = "..."
  secret_key   = "..."
  facebook_url = "https://graph.facebook.com"

  OAuth2::Client.new(api_key, secret_key, site: facebook_url)
end


def redirect_uri
  url('/facebook/callback')
end