libr-2/app/workers/user_avatar_worker.rb

class UserAvatarWorker
  include Sidekiq::Worker

  sidekiq_options queue: :avatar, retry: 3

  def perform user_id
    user = User.find user_id
    unless user.avatar
      url = "https://robohash.org/#{user.name}.png?set=set#{rand(4)}"
      user.create_avatar file: URI.parse(url)
    end
  end

end