A rack middleware that redirect URLs that have ID to URLs that have SLUG. Should run well with typical friendly_id setup.

redirect_old_url.rb

class RedirectOldUrl
  RESOURCES_WITH_SLUGS = [
    "conferences",
    "users",
    "events",
    "articles",
    "partners"
  ]

  def initialize(app)
    @app = app
  end

  def call(env)
    @request = Rack::Request.new(env)

    if env["HTTP_ACCEPT"].include?("text/html") && is_old_url?
      [301, {"Location" => "http://#{host}#{new_path}"}, []]
    else
      @app.call(env)
    end
  end

  def host
    (@request.port == 80) ? @request.host : @request.host_with_port
  end

  def new_path
    id = @route_params[:id]
    model = @route_params[:controller].classify.constantize
    slug = model.find(id).slug || id
    @route_params[:id] = slug
    @route_params[:only_path] = true
    Rails.application.routes.url_for(@route_params)
  end

  def is_old_url?
    @route_params = Rails.application.routes.recognize_path(@request.path)
    # Check if ID is an integer and controller is on list
    !!(@route_params[:id] =~ /^[-+]?[0-9]+$/) && RESOURCES_WITH_SLUGS.include?(@route_params[:controller])
  end
end