The NetBeans IDE created by Sun Microsystems is often associated with Java development. However, in the past few years, Sun has slowly been adding support for more languages to NetBeans. Ruby (and more specifically, JRuby) is one of those languages.
Combined with the MySQL database, NetBeans provides a complete Rails and Ruby on Rails environment.
It's not necessary for you to have Ruby installed at all to use Ruby on NetBeans. NetBeans comes with JRuby, which may suit your needs just fine. If, however, it doesn't, you can point NetBeans to a native Ruby installed on your system.