title: Having Trouble Using "brew link" to switch between PostgreSQLs
body: <p>I want to run a Rails app on Heroku so I installed PostgreSQL on my Snow Leopard machine using Homebrew. Currently, brew installs 9.1.2 into /usr/local/Cellar/postgresql.  After some research, I discovered that the free, shared version of Heroku requires 8.3.x.  While homebrew-alt doesn't have 8.3, it does have 8.4 so I downloaded it instead and it installed into /usr/local/Cellar/postgresql8. I then confirmed that I am able to create and run databases on each version.  Thinking that I could use "brew [link | unlink] to switch back and forth, I ran "brew unlink postgresql8" and then "brew link postgresql".  However, running the latter command results in this error:</p>
  
  <p>Linking /usr/local/Cellar/postgresql/9.1.2...
  Error: Cannot link postgresql
  Another version is already linked: /usr/local/Cellar/postgresql/9.1.2</p>
  
  <p>Is there any way to do this (switch back and forth) or am I misunderstanding how brew link/unlink is supposed to be used?</p>
  
  <p>By the way, "brew list" shows "postgresql" and "postgresql8" so I don't think it's the case that I should say "brew link postgresql9".</p>
  
  <p>Thanks.</p>