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// CreateOrder creates an order for an album and returns the new order ID. => CreateOrder 为 album 创建一个订单，并返回新的订单 ID。
func CreateOrder(ctx context.Context, albumID, quantity, custID int) (orderID int64, err error) {

    // Create a helper function for preparing failure results.
    fail := func(err error) (int64, error) {
        return fmt.Errorf("CreateOrder: %v", err)
    }

    // Get a Tx for making transaction requests.
    tx, err := db.BeginTx(ctx, nil)
    if err != nil {
        return fail(err)
    }
    // Defer a rollback in case anything fails.
    defer tx.Rollback()

    // Confirm that album inventory is enough for the order. => 确认专辑有足够的库存。
    var enough bool
    if err = tx.QueryRowContext(ctx, "SELECT (quantity >= ?) from album where id = ?",
        quantity, albumID).Scan(&enough); err != nil {
        if err == sql.ErrNoRows {
            return fail(fmt.Errorf("no such album"))
        }
        return fail(err)
    }
    if !enough {
        return fail(fmt.Errorf("not enough inventory"))
    }

    // Update the album inventory to remove the quantity in the order.
    _, err = tx.ExecContext(ctx, "UPDATE album SET quantity = quantity - ? WHERE id = ?",
        quantity, albumID)
    if err != nil {
        return fail(err)
    }

    // Create a new row in the album_order table.
    result, err := tx.ExecContext(ctx, "INSERT INTO album_order (album_id, cust_id, quantity, date) VALUES (?, ?, ?, ?)",
        albumID, custID, quantity, time.Now())
    if err != nil {
        return fail(err)
    }
    // Get the ID of the order item just created.
    orderID, err := result.LastInsertId()
    if err != nil {
        return fail(err)
    }

    // Commit the transaction.
    if err = tx.Commit(); err != nil {
        return fail(err)
    }

    // Return the order ID.
    return orderID, nil
}