Order creation — stock + transaction

export async function createOrder(
  userId: number,
  items: { productId: number; qty: number }[],
) {
  const order = await prisma.$transaction(async (tx) => {
    const lines = [];

    for (const item of items) {
      const product = await tx.product.findUnique({
        where: { id: item.productId },
      });

      if (!product || product.qty < item.qty) {
        throw new Error("insufficient stock - " + item.productId);
      }

      await tx.product.update({
        where: { id: item.productId },
        data: { qty: { decrement: item.qty } },
      });

      lines.push({
        productId: item.productId,
        qty: item.qty,
        price: product.price,
      });
    }

    const total = lines.reduce(
      (sum, l) => sum + Number(l.price) * l.qty,
      0,
    );

    return tx.order.create({
      data: {
        userId,
        total,
        items: { create: lines },
      },
    });
  });

  return order;
}