How to express SQL subqueries, EXISTS, and NOT EXISTS correctly in ActiveRecord, and why naive translations often return incorrect results.
A 5-table ActiveRecord join took 8 seconds. Rewriting it as a subquery took it to 200ms. Here's what changed and why the query planner behaves differently.
N+1 queries aren't a Rails flaw — they come from how you shape associations. Learn to spot the pattern early and fix it without over-eager loading.
ActiveRecord isn't SQL with Ruby syntax — it's a different mental model. Here's why the shift trips up SQL-fluent developers and how to make it click.