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feat: discount coupons with usage limits, expiry and minimum cart value - #3
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Adds percent and fixed-amount coupons. Admins create, list and deactivate them; customers apply one coupon per cart. Usage is counted inside the checkout transaction and returned when an order is cancelled. VAT and the free-shipping threshold are calculated on the discounted amount. Orders store the discount and coupon code (migration 2). Adds 9 tests. Closes #2 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01FYiLwGkrgfn7ftZh1WDFpq
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| const coupon = findCoupon(db, row.code); | ||
| return coupon && coupon.isActive ? coupon : null; |
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If a coupon expires after being applied, or another customer consumes its last permitted use, appliedCoupon still returns it because only isActive is checked. The cart view passes this result directly to calculateTotals, so GET /cart continues advertising a discount and total that checkout rejects with a 409. Check expiry and remaining uses when calculating the cart view, and expose the unavailable coupon without presenting its discount as usable.
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| const discount = coupon ? couponDiscount(coupon, subtotal) : 0; | ||
| const net = subtotal - discount; | ||
| // A discount lowers the taxable amount, so the VAT inside it shrinks proportionally. | ||
| const vatTotal = subtotal === 0 ? 0 : Math.round((grossVat * net) / subtotal); |
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When the original VAT requires rounding, scaling the already-rounded grossVat introduces a second rounding error instead of recalculating VAT on the discounted amount. For example, a single item priced at 3 kuruş with 20% VAT and a fixed 1-kuruş discount produces vatTotal: 1, although vatIncluded(2, 20) is 0. This incorrect value is persisted on the order. Allocate the discount to taxable lines and calculate VAT from their discounted amounts before rounding, rather than scaling the rounded original VAT.
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Stores run campaigns with codes like
HOSGELDIN10; the API had no way to model them.What changed
GET /coupons,POST /coupons,DELETE /coupons/:code(deactivate, past orders keep their data)PUT /cart/coupon,DELETE /cart/coupon; the cart shows the applied coupon anddiscountUPDATE … WHERE used_count < max_uses), so it cannot be over-usedcoupons,cart_coupons,orders.discount,orders.coupon_codeCloses #2