config.ru

require_relative 'boot'
db = Sequel.connect(ENV['DATABASE_URL'])
Urls.attach_db(db[:urls])
run UrlShortener

url_shortener.rb

require 'sinatra/base'
require 'uri'
require_relative 'urls'

class UrlShortener < Sinatra::Application

  INVALID_URL_ERROR = 'Please+enter+a+valid+URL'

  get '/' do
    error = params[:error]
    url_to_shorten = params[:url]
    erb :index, :locals => {:error => error, :url_to_shorten => url_to_shorten}
  end

  post '/shorten' do
    url_to_shorten = params[:url]
    if url_to_shorten.empty? || url_to_shorten.split(' ').count > 1 || !is_valid_url?(url_to_shorten) || url_to_shorten.nil?
      redirect "/?url=#{url_to_shorten}&error=#{INVALID_URL_ERROR}"
    else
      begin
        id = Urls.create(url_to_shorten)
        redirect to("/#{id}?stats=true")
      rescue Sequel::AlreadyExistsError
        redirect "/?url=#{url_to_shorten}&error=This+already+exists"
      end
    end

  end

  get '/:id' do
    stats = params[:stats] == 'true'
    id = params[:id]
    url_container = Urls.find(id)
    new_url = "#{request.base_url}/#{id}"
    if stats
      erb :stats, :locals => {url_container: url_container, new_url: new_url} #{:original_url => original_url, :, :views => views}
    else
      url_container.increase_views
      redirect original_url
    end
  end

  private
  def is_valid_url?(url_to_shorten)
    url = false
    if url_to_shorten.empty? || url_to_shorten.split(' ').count > 1 || url_to_shorten.nil?
    elsif url_to_shorten
      if url_to_shorten =~ /^#{URI::regexp}$/
        url = true
      end
      url
    end
  end

end

urls.rb

class Urls
  class << self
    attr_accessor :db

    def attach_db(db)
      @db = db
    end

    def create(url, vanity_name = nil)
      if vanity_name
        @db.insert(url: url, vanity_name: vanity_name)
        vanity_name
      else
        # wrap in a transaction
        id = @db.insert(url: url)
        @db.where(id: id).update(vanity_name: id.to_s)
        id.to_s
      end      
    end

    def find(id)
      new(@db.where(vanity_name: id))
    end
  end

  attr_accessor :id, :stats, :url, :vanity_name

  def initialize(record)
    @record = record

    data = record.first
    @id = data[:id]
    @stats = data[:stats]
    @url = data[:url]
    @vanity_name = data[:vanity_name]
  end

  def increase_views
    @record.update(:stats => Sequel.+(:stats,1))
  end
end