I encountered two issues and learned two quick lessons when deploying a Merb
application I have been working on. Aside from the fact that you can use
to get an array of subdomains, you must do two little things to get this to
play nice when your application is deployed upstream on Nginx (and likely
apache, too) as well as handling the same situation in local development.
First, in your Nginx configuration file be sure to set the proper header for
host, otherwise the host that Merb will see is the name you give to the upstream
section of your configuration. To pass this information on correctly, do this:
proxy_set_header Host $host;
inside the "location" block for your application's root.
Second, in order to get the right subdomains, Merb expects you are using a TLD or top-level-domain, such as example.com. If you, like me, are using a local hostname, such as localhost, you aren't going to get anything when you ask for subdomains. So, in /etc/hosts I simply set up a domain pointing to 127.0.0.1 for a place I know I'll never go.
I hope these two little tips will help you to avoid some of the trouble I had to deal with, even if it was only 5 minutes of my time, or yours.