link-shortener-rails.rb

# rails new shrtn
# cd shrtn

# rails g model Site url:uniq slug:index
# rake db:migrate

# site.rb
class Site < ApplicationRecord
  after_create :generate_slug
  delegate :url_helpers, to: 'Rails.application.routes'
  
  def to_param
    slug
  end

  def short_url
    url_helpers.site_url(self)
  end

  private

  def generate_slug
    length = 3
    loop do
      slug = to_sha(length)
      return self.slug = slug unless Site.find_by(slug: slug)
      length = length + 1
    end
  end

  def to_sha(size = nil)
    sha = Digest::SHA256.hexdigest("#{url}:#{created_at}")
    return sha.slice(0,size) if size
    sha
  end
end

# sites_controller.rb
class SitesController < ApplicationController
  
  def get
    set_site
    redirect_to @site.url
  end
  
  def create
    @site = Site.where(url: params[:url]).first_or_create
    render text: @site.short_url
  end
  
  private
  
  def set_site
    @site = Site.find_by! slug: params[:slug]
  end
end

# routes.rb
Rails.application.routes.draw do
  post 'sites/:url', to: 'sites#create'
  get '/:slug', to: 'sites#get', as: :site
end