Here's how I was able to scale my Parellels VM to work with a retina display.
I started a trial of Parellels Desktop to run Windows 7 on my retina MacBook Pro. Every time I resized the screen, Windows 7 resized itself, and everything was super, super small. I did some research and found that there may not be a tried and true answer, but here's what I did, and now Windows 7 looks awesome on my screen:
I hope this works for you, too!
Here are a couple methods for dealing with uni-directional many-to-many associations in Rails.