gistfile1.txt

 Get the .dmg

There is a one-click installer available for PostgreSQL on Mac OS X. It packages the database up as a .dmg file. It is available here: http://www.enterprisedb.com/products/pgdownload.do#osx
Edit your PATH

The .dmg does the installation and setup, but it doesn't alter your PATH variable to include the PostgreSQL tools. To do this, edit

~/.bash_profile

and add the lines

PATH="/Library/PostgreSQL/8.4/bin/:${PATH}" export PATH

Installing the postgres rubygem for Rails

Once you have the binaries in your path, installing the postgres driver for ruby is simple. Just run

sudo gem install pg

This should compile the native extensions using pg_config for the required environment variables. Once done, you can access a PostgreSQL database from Ruby, or set it as the adapter for a Ruby on Rails database.