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The PR

Title: Add support for multi-use discount codes Summary: Currently, discount codes are single-use—once applied to an order, they’re marked as used and can’t be applied again. This PR adds support for multi-use codes. Merchants can now create discount codes that multiple customers can use. Changes:
  • New multi_use flag on the Discount model
  • Modified checkout logic to skip the “mark as used” step if multi_use == true
  • Database migration to add the flag

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Factor Analysis

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Verdict

REVIEW_REQUIRED
The PR introduces discount logic that requires validation of business rules before merging.

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Merge Blockers

Question 1: If an expired discount code were applied today, would the system still charge the customer? The PR’s logic checks if multi_use == true before marking a code as used. But it doesn’t verify the code’s expiration date. If a multi-use code has expired, it’s still applied. How to respond: Verify that expiration checks happen before the code is applied. Show the code that validates this.
Question 2: What happens if a merchant changes a code from single-use to multi-use while orders are being processed? If an order is in flight when the flag changes, it might skip the “mark as used” step even though the merchant intended it to be single-use. How to respond: Either:
  • Explain why this race condition is acceptable (e.g., grace period is fine)
  • Show code that prevents the race condition (e.g., snapshot the flag at order creation)
  • Add a test that verifies your chosen behavior
Question 3: Can a customer apply the same multi-use code multiple times to different orders in the same session? If the code is valid, nothing in this PR prevents reuse. If the intent is “once per customer,” that validation is missing. How to respond: Clarify the business rule. If it’s “unlimited uses,” no change needed. If it’s “once per customer,” add validation.

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Potential Failure Scenarios

Scenario 1: Expired multi-use code applied
  • When: Customer applies a multi-use discount code that has expired
  • How it unfolds:
    1. Code is in the system and flagged multi_use = true
    2. Code’s expiration date has passed
    3. Checkout logic skips expiration check (gap in this PR)
    4. Code is applied, discount calculated
    5. Order processed, customer charged less than they should be
  • What breaks: Revenue leakage; accounting gap; customer might not notice or might exploit it
Why it matters: A single expired code could be applied to hundreds of orders if it’s a popular discount.
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