activerecord

Avoid writing rows when nothing changed—especially in batch jobs. Check changed? or compute the would-be update and skip if identical. This reduces bloat and autovacuum pressure.

Rails migrations evolve database schema over time. I use change method for reversible migrations. Migrations create tables, add/remove columns, add indices. up and down methods provide explicit control. Irreversible migrations like data transformation

If multiple admins edit the same record, use lock_version. Rails will raise on conflicting updates, and you can show a friendly “this changed underneath you” message. It prevents subtle lost updates.

Callbacks hook into the ActiveRecord lifecycle to execute code before or after operations like create, update, or destroy. I use before_validation to normalize data (like downcasing emails), after_create to trigger welcome emails, and before_destroy t

Race conditions happen. The correct “find or create” in production uses a unique constraint and a retry on conflict, not a naive check-then-insert. Let the database serialize the race.

Avoid loading whole records when you only need to know if something exists. exists? is good; for complex joins, a scoped select(1).limit(1) can be clearer and keeps the DB workload low.

I stopped doing row-by-row imports once they started hammering the DB. In Migration (unique index), I added a real uniqueness guarantee on provider + uid, because bulk writes only stay correct if the database can enforce constraints. Then, in Bulk ups

If you sort by a non-unique column (score, created_at), pagination can “skip” or “duplicate” records. Always add a secondary unique key like id for deterministic ordering.

When you need to invalidate caches by changing timestamps, use touch_all to avoid per-record callbacks. It’s fast, explicit, and doesn’t run unintended side effects.

OFFSET gets slower as tables grow and becomes inconsistent under writes. Keyset pagination is stable and fast: paginate by (created_at, id) cursor. This is a common “senior Rails” upgrade for activity feeds.

Backfills often fail because we accidentally load full records and associations. Use select to fetch only needed columns and find_each to keep memory flat. This is basic, but it’s where outages come from.

Heavy work inside a transaction increases lock time and deadlock risk. Use after_save_commit to schedule slow tasks (thumb generation, external sync) once the write is durable.