This article assumes you have already setup your SSL cert on plesk (plenty of articles on that). Also, assumes you have running an existing Rails app on non-secure HTTP. We need to setup our vhost and vhost_ssl conf files that PLESK creates. Mine are located at:

/var/www/vhosts/domain.com/conf/

In there folder you should see:

httpd.include  vhost.conf  vhost_ssl.conf

If you don’t see the two vhost conf files that’s fine, just create them and add the following lines to both:

ServerName domain.com
DocumentRoot "/var/www/vhosts/domain.com/httpdocs/app/public"

This is saying point both ssl and non ssl to the same Rails app public folder. Now we need update plesk to show the new vhost conf and restart apache:

sudo /usr/local/psa/admin/sbin/websrvmng -a

and

sudo /etc/init.d/httpd restart

Now if you go to https://domain.com your site should be loaded just fine and shows that SSL is being used