When you try and install the native mysql gem you get:

>sudo gem install mysql

Building native extensions.  This could take a while…

ERROR:  Error installing mysql:

ERROR: Failed to build gem native extension.

checking for mysql_query() in -lmysqlclient… no

/usr/bin/ruby1.8 extconf.rb install mysql

blah..blah…and it fails.

Most likely you installed the mysql server package but you also need to install the mysql client development libraries (check the name of the latest package):

sudo apt-get install libmysqlclient15-dev

Now try again:

>sudo gem install mysql

Successfully installed mysql-2.7
1 gem installed

Building native extensions.  This could take a while…

That’s better.