Quick Reference

Critical Rules

• save returns false on failure — save! raises, check return or use bang
• update_all/delete_all skip callbacks and validations — data corruption if unaware
• find_each for batches — Model.all.each loads entire table into memory
• redirect_to doesn't halt execution — code after it runs, use and return
• dependent: :destroy missing — orphan records accumulate forever
• default_scope pollutes all queries including joins — almost always wrong
• Callbacks chain silently — throw :abort stops save but returns false, not exception
• includes without references in where string — N+1 still happens
• ||= memoization caches nil/false — use defined?(@var) ? @var : @var = compute
• has_many through: vs has_and_belongs_to_many — latter has no join model for attrs
• Nested before_action — multiple inheritance makes flow unreadable
• render doesn't stop action — code continues, duplicate render crashes