This is a should guide of knowledge, where I demonstrate some issues and offer solution for them.

Ruby on Rails developer fails with fixes.md

any record exists

bad

  @any_user = User.all.any?        # SELECT `users`.* FROM `users`
  @any_user = User.scoped.present? # SELECT COUNT(*) FROM `users`

• fetch all data from database
• load memory for all needed objects

good

@any_user = User.exists? # SELECT 1 AS one FROM `users` LIMIT 1

• efective
• quick
• readble

sorting LIFO

bad

  User.order("created_at DESC")

• you need additional index for timestamp column
• if two record where added at the same second, result will not be correct

good

  User.order("id DESC")

• using primary key
• correct result if two record where added at the same second