Ruby gem error installing rack-mount + Rails

Question

I would like to install Rails 3.0 Beta but ran into what appears to be a dependency error:

$: sudo gem install rack-mount
Successfully installed rack-mount-0.5.1
1 gem installed
Installing ri documentation for rack-mount-0.5.1...
Installing RDoc documentation for rack-mount-0.5.1...



$: sudo gem install rails --prerelease
ERROR:  Error installing rails:
    actionpack requires rack-mount (~> 0.4.0, runtime)

Why is it unable to recognize that rack-mount is already installed?

Answer 1

As noted in the comments here, you need rack-mount at exactly version 0.4.0. Install with the following command:

gem install rack-mount -v 0.4.0