How do I use rack rewrite to add www to a url?

Question

I want to redirect my rails app to add www to the url if the user doesn't use it. I found this site to remove it, but I would like to do the opposite:

http://steve.dynedge.co.uk/2010/03/03/using-rackrewrite-to-remove-the-www/

Answer 1

Can you not just do something like this?

  r301 /.*/,  Proc.new {|path, rack_env| "http://www.#{rack_env['SERVER_NAME']}#{path}" },
    :if => Proc.new {|rack_env| ! (rack_env['SERVER_NAME'] =~ /www\./i)}