Challenge 2: Write the Checkout Transaction — Possible Solution
====================================================================

const session = client.startSession();

try {
  await session.withTransaction(async () => {
    await orders.insertOne(
      {
        _id: new ObjectId(),
        product_id: "P200",
        customer_id: "C104",
        quantity: 1,
        status: "placed"
      },
      { session }
    );

    await products.updateOne(
      { _id: "P200" },
      { $inc: { stock: -1 } },
      { session }
    );
  });
} finally {
  session.endSession();
}

WHY THIS WORKS AS AN ANSWER
------------------------------
startSession() creates the session both operations will share, and
endSession() runs in a finally block so the session is always cleaned
up, whether the transaction committed or was aborted.

withTransaction() wraps the callback: if BOTH insertOne and updateOne
succeed, it commits automatically; if either one throws (for example,
a driver-level error, or application logic that rejects the order
because stock is already 0), it aborts the WHOLE transaction
automatically — the order insert is rolled back even though it may
have technically already run and "succeeded" in isolation.

Every single operation inside the callback passes { session } as its
options argument — this is what actually ties each write to the
transaction. Forgetting it on either call (an easy mistake) would mean
that operation runs OUTSIDE the transaction, immediately and
permanently, regardless of what happens to the rest of the block.

Using withTransaction() here rather than manual
startTransaction()/commitTransaction()/catch/abortTransaction() also
means transient errors (e.g. a brief replica set election) are retried
automatically, per this chapter's closing gotcha.