I was advised to use RVM to manage Ruby on the Mac.

1. Sign up as an Apple developer
2. Download and install XCode
3. Follow the instructions for installing RVM.
   1. Install RVM by executing $ bash < <( curl http://rvm.beginrescueend.com/releases/rvm-install-head ) in the terminal window.
   2. I didn’t have one, so I had to create a .bash_profile file by following the instructions at redfinsolutions.com:
      1. Start up Terminal
      2. cd ~/ (to go to your home folder)
      3. touch .bash_profile to create the file.
      4. open -e .bash_profile to open it in TextEdit.
   3. Paste [[ -s "$HOME/.rvm/scripts/rvm" ]] && . "$HOME/.rvm/scripts/rvm" # This loads RVM into a shell session. into the file and save
   4. Close the Terminal window, open another one, and type $ type rvm | head -1. If everything has worked, you should see rvm is a function
      Then you can run
      • rvm install 1.9.2
      • rvm use 1.9.2

Next you follow the instructions to set up rvm with TextMate. In order to run the script,

1. Create the file in the same way as creating the .bash_profile file, naming it [myfilename].command
2. Execute chmod u+rwx [myfilename].command to give yourself read-write-execute permissions
3. Double-click on it in Finder to execute it.

Finally, install the pg gem:

1. Find out the pg_config path by executing mdfind pg_config|grep bin|uniq
2. Install the gem using gem install pg -- --with-pg-config=[the path returned by the previous line]

Notice that sudo isn’t used, because RVM is being used.

See this tutorial for setting up a Rails app with a PostgreSQL DB.