Problems understanding a Ruby example file for Google API Client Authentication - ruby-on-rails

Question

I am trying to access Google APIs from my Rails App.
I understand the basic flow for OAuth2.0 for Web Applications.
And I consider myself having a basic understanding of Ruby and Rails.
Yet, I can't make sense of the example Google shows in the "API Client Library for Ruby (Alpha)" Guides.
require 'google/apis/calendar_v3'
require 'google/api_client/client_secrets'
require 'sinatra'
require 'logger'
enable :sessions
def logger; settings.logger end
def calendar; settings.calendar; end
def user_credentials
# Build a per-request oauth credential based on token stored in session
# which allows us to use a shared API client.
#authorization ||= (
auth = settings.authorization.dup
auth.redirect_uri = to('/oauth2callback')
auth.update_token!(session)
auth
)
end
configure do
log_file = File.open('calendar.log', 'a+')
log_file.sync = true
logger = Logger.new(log_file)
logger.level = Logger::DEBUG
Google::Apis::ClientOptions.default.application_name = 'Ruby Calendar sample'
Google::Apis::ClientOptions.default.application_version = '1.0.0'
calendar_api = Google::Apis::CalendarV3::CalendarService.new
client_secrets = Google::APIClient::ClientSecrets.load
authorization = client_secrets.to_authorization
authorization.scope = 'https://www.googleapis.com/auth/calendar'
set :authorization, authorization
set :logger, logger
set :calendar, calendar_api
end
before do
# Ensure user has authorized the app
unless user_credentials.access_token || request.path_info =~ /^\/oauth2/
redirect to('/oauth2authorize')
end
end
after do
# Serialize the access/refresh token to the session and credential store.
session[:access_token] = user_credentials.access_token
session[:refresh_token] = user_credentials.refresh_token
session[:expires_in] = user_credentials.expires_in
session[:issued_at] = user_credentials.issued_at
end
get '/oauth2authorize' do
# Request authorization
redirect user_credentials.authorization_uri.to_s, 303
end
get '/oauth2callback' do
# Exchange token
user_credentials.code = params[:code] if params[:code]
user_credentials.fetch_access_token!
redirect to('/')
end
get '/' do
# Fetch list of events on the user's default calandar
events = calendar.list_events('primary', options: { authorization: user_credentials })
[200, {'Content-Type' => 'application/json'}, events.to_h.to_json]
end
Of course this looks like Ruby, but I never saw some of the constructs used here, and I am not sure where in a Rails app that code would go.
Is it Ruby? If so, how do I use it?