gem install mysql2
出现下面错误： Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
checking for rb_thread_blocking_region()... yes
checking for rb_wait_for_single_fd()... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lm... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lz... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lsocket... no
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lnsl... yes
checking for mysql_query() in -lmysqlclient... no
checking for main() in -lmygcc... no
checking for mysql_query() in -lmysqlclient... no
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of
necessary libraries and/or headers. Check the mkmf.log file for more
details. You may need configuration options.
An error occurred while installing mysql2 (0.3.11), and Bundler cannot continue.
Make sure that `gem install mysql2 -v '0.3.11'` succeeds before bundling.
此问题是没有安装 libmysqlclient-dev 造成的，所有安装 libmysqlclient-dev 即可：
apt-get install libmysqlclient-dev