ActiveRecord may be both the best and worst part of Rails. Currently the largest scaling problem that I'm facing is with all the before_* and after_* callbacks which run per model record rather than a batch of changed records. This is an N+1 query landmine field for which ActiveRecord offers no solutions.

AR is the worst thing about Rails - it's anti-pattern central. The Ruby REPL is amazing, however.

ActiveRecord or how to badly couple your storage with your objetcs.