If you get a stacktrace complaining about uninitialized constant MysqlCompat::MysqlRes a system library update might broke your gem.
You might have switched from MySQL to MariaDB, but forgot to rebuild your MySQL gems.
Try fully removing and re-installing the gem:
gem uninstall mysql mysql2
bundle install
Posted by Thomas Eisenbarth to makandra dev (2012-11-14 17:23)