add-lib_canonical_redirect.rb

class CanonicalRedirect
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
#
#
#
# CHANGE THE PRODUCTION DOMAIN WHERE IT SAYS PRODUCTION_DOMAIN_HERE
#
#
#

    if request.host =~ /^PRODUCTION_DOMAIN_HERE.com/
      [301, {"Location" => request.url.sub("//", "//www.")}, self]
    else
      @app.call(env)
    end
  end

  def each(&block)
  end
end

update-config_application.rb

config.autoload_paths += %W(#{config.root}/lib)
config.middleware.use "CanonicalRedirect" # add inside class Application < Rails::Application