This is just a tiny rails application demonstrating how you can set up active record associations that span several models to create a 'short cut'.
If you want to run the code, run the following:
bundle install
bin/rake db:setup
bin/rails console
The seeds.rb file will set you up with a few orders and a user. You can then play around with the models to get from a user to the products they've ordered, or from an order to it's products.
For more info check out this blog post I wrote.