Use links in your JSON to make your Ember app smarter.
Speed up your tests by telling Capybara WebKit not to hit external APIs.
Take your queries to the next level with window functions.
Use SQL EXPLAIN to understand when and why indexes are used (or not).
Sleep better at night knowing your migrations are reversible.
Calculations across objects are ripe for using a map and an inject but frequently, with ActiveRecord, the solution is more elegant and faster if we can have SQL do the work instead.