Comment 33 for bug 145267

I had a look at your implementation and it looks interesting. However I wonder if the usage of the update-alternative system is necessary as it brings additional complexity (as already pointed by Lucas above).

Another option would be to simply force symlink binaries from /var/lib/ruby1.*/bin/ to /usr/local/bin/, that way the last installed gem always wins.

The main point being that the gem command should installed everything in /usr/local/ - it can be considered as a program installed by the local administrator. It should not touch anything in /usr (which is reserved for dpkg).

So rather than trying to figure out ways to make dpkg and gem know about each other, it seems easier to make sure that they won't interfere (dpkg works in /usr, gem in /usr/local).

If you think that update-alternatives is still worth using, I'd suggest you use the --altdir and --admindir options, so that you don't step on the update-alternatives system configuration (using /usr/local/etc/gems/alternatives and /usr/local/lib/gems/alternatives respectively). Going down that road, it may also be useful to extend the gem command to provide a way to switch from one alternative to the other so the end user won't have to learn about the update-alternative system.