This is an old problem that affects an old Ruby gem: mysql (2.8.1). Apparently, it has a little bug somewhere that makes it look for a file in the wrong folder. I get this error always after installing a new app. The last time has been some minutes ago, after installing TracksApp 2.1, which is based on the old Rails 2.3.
The MySQL page about Ruby adapters speaks about two of them. One called MySQL/Ruby, and the other (after a huge name creation effort)… Ruby/MySQL. They are respectively provided by the gems mysql (2.8.1) and ruby-mysql (2.9.10). As you may have noticed, the first gem is the old version of the second. That is also confirmed by the dates they got updated, which is: 2.8.1, on 2009-08-21 and 2.9.10, on 2012-07-12.
Both adapters are authored by Tomita Masahiro. I do appreciate the effort he did, and his generosity in making them available to me, no doubt about that. But in my opinion, the name change was a mistake. I suppose he did it to tell the world they were two very different adapters (the old one C-based and the new one Ruby-based). Nonetheless, they really were two subsequent versions of the same thing.
The problem with TracksApp is exactly that. If Tomita Masahiro had stuck to the mysql name, then gem “mysql” in TracksApps would have referenced the latest version (what today is instead ruby-mysql) and all would have worked fine. But now, even with ruby-mysql installed, that gem requirement in TracksApp makes my system install the old mysql gem. That in turn makes Rails prefer mysql over ruby-mysql (why? lexical order?) and due to the known bug all my apps break.
My solution is to manually uninstall mysql each time it gets installed by some app. In fact I don’t know how to make my global Rails environment ignore mysql (2.8.1) even if some app requires it. I’ve tried with “bundle install –without mysql”, but mysql is a gem, not a group, so it does not work… My fix works because ruby-mysql perfectly replaces mysql and it’s just a drop-in, sharing the same adapter name (mysql).
Clearly, TracksApp is a new version of an old software, even if it is still based on Rails 2.3. If I was developing it, I’d have updated the Gemfile…