Installing Datamapper on Ubuntu 9.04, I got this mysterious error:

$ sudo gem install do_mysql
Building native extensions.  This could take a while...
ERROR:  Error installing do_mysql:
ERROR: Failed to build gem native extension.

/usr/bin/ruby1.8 extconf.rb install do_mysql
extconf.rb:3:in `require': no such file to load -- mkmf (LoadError)
from extconf.rb:3

After much googling and trying other solutions like installing libmysqlclient16-dev, I stumbled upon Al Hoang's related problem and solution:

$ sudo aptitude install ruby-dev

Then

$ sudo gem install do_mysql
Building native extensions.  This could take a while...
Successfully installed do_mysql-0.10.0
1 gem installed

Installing libmysqclient16-dev could have anticipated a problem that would have arisen later--it may or may not be necessary.