dragonfly_config.rb

# https://viget.com/articles/the-right-way-to-store-and-serve-dragonfly-thumbnails

Dragonfly.app.configure do
  # ... (other settings)
  
  define_url do |app, job, opts|
    thumb = Thumb.find_by_signature(job.signature)

    if thumb
      app.datastore.url_for(thumb.uid)
    else
      app.server.url_for(job)
    end
  end

  before_serve do |job, env|
    thumb = Thumb.find_by_signature(job.signature)

    if thumb
      throw :halt, [301, { 'Location' => job.app.remote_url_for(thumb.uid) }, [""]]
    else
      uid = job.store
      Thumb.create!(uid: uid, signature: job.signature)
    end
  end
end