Transactions trong Active Record là gì? Khi nào cần dùng?

Transaction đảm bảo tất cả hoặc không có thao tác DB nào được commit — nếu bất kỳ bước nào fail, toàn bộ rollback.

ActiveRecord::Base.transaction do
  sender.update!(balance: sender.balance - amount)
  receiver.update!(balance: receiver.balance + amount)
  Transfer.create!(from: sender, to: receiver, amount: amount)
end
# Nếu receiver.update! raise → sender.balance cũng bị rollback

Transaction tự động bao quanh mỗi save / create / destroy. Cần explicit transaction khi:
- Nhiều thao tác DB phụ thuộc nhau (chuyển tiền, đặt hàng + trừ kho).
- Dùng save! (raise) để trigger rollback thay vì return false.

Lưu ý: callback after_commit chạy sau khi transaction commit thành công — dùng cho side-effects để không trigger khi rollback.

A transaction guarantees all-or-nothing DB operations — if any step fails, everything is rolled back.

ActiveRecord::Base.transaction do
  sender.update!(balance: sender.balance - amount)
  receiver.update!(balance: receiver.balance + amount)
  Transfer.create!(from: sender, to: receiver, amount: amount)
end
# If receiver.update! raises → sender.balance is also rolled back

Every save / create / destroy is automatically wrapped in a transaction. You need an explicit one when:
- Multiple DB operations depend on each other (money transfers, order + inventory deduction).
- Using save! (raises) to trigger rollback instead of returning false.

Note: the after_commit callback fires only after a successful commit — use it for side-effects to avoid triggering them on rollback.