Resolution

Since version 8.1 Plesk Control Panel supports Ruby on Rails. There is no an option in Plesk Control Panel that should be  checked to enable Ruby on Rails support, it is only enough to get ruby packages installed.

To install on your site an application written in Ruby:

1. Go to Domains > your-domain.com > Setup.
2. Select the CGI and FastCGI check boxes, and click OK.
3. Connect to your FTP account, change to the /htdocs directory, and create a subdirectory where the application files will reside. Let's call this directory your_application, for illustration purposes.
4. Upload the application files to the htdocs/your_application directory.
5. Create a file with name .htaccess in this directory, open it with a text editor of your choice and add the following lines into the file:

RewriteEngine On RewriteRule ^$ /public/index.html [L] RewriteCond %{REQUEST_URI} !^/your_application/public
RewriteRule ^(.*)$ /public/$1 [L] RewriteCond %{REQUEST_FILENAME} !-f RewriteRule ^(.*)$ public/dispatch.fcgi/$1 [QSA,L]

6. Save the file.
7. Remove the file your_application/public/.htaccess
8. Open the file your_application/public/dispatch.fcgi with a text editor and put the following lines there: #!/usr/bin/ruby
9. Save the file.

Now the web application will be accessible at the following URL: http://your-domain.com/your_application.

Additional information

Refer to Plesk documentation for details about installation of a Ruby on Rails application.

http://download1.swsoft.com/Plesk/Plesk8.4/Doc/en-US/plesk-8.4-unix-administrators-guide/index.htm
Serving Your Customers -> Hosting Web Sites ->  Installing Applications ->  Installing Ruby Web Applications