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Using prepared statements with the static interface (C++14)

This example demonstrates how to use prepared statements with the static interface to implement a minimal order management system for an online store.

The example employs synchronous functions with exceptions as error handling. See this section for more info on error handling.

This examples requires you to run example/order_management/db_setup.sql. You can find table definitions there.

/**
 * This example implements a very simple command-line order manager
 * for an online store, using prepared statements. You can find the table
 * definitions in example/order_management/db_setup.sql. Be sure to run this file before the example.
 * This example assumes you are connecting to a localhost MySQL server.
 *
 * The order system is intentionally very simple, and has the following tables:
 *  - products: the list of items our store sells, with price and description.
 *  - orders: the main object. Orders have a status field that can be draft, pending_payment or complete.
 *  - order_items: an order may have 0 to n line items. Each item refers to a single product.
 *
 * Orders are created empty, in a draft state. Line items can be added or removed.
 * Orders are then checked out, which transitions them to pending_payment.
 * After that, payment would happen through an external system. Once completed, an
 * order is confirmed, transitioning it to the complete status.
 * In the real world, flow would be much more complex, but this is enough for an example.
 *
 * We'll be using the static interface to retrieve results from MySQL.
 * This makes use of the static_results<RowType> class template.
 * To use it, we need to define a set of structs/tuples describing the shape
 * of our rows. Boost.MySQL will parse the received rows into these types.
 * The static interface requires C++14 to work.
 *
 * Row types may be plain structs or std::tuple's. If we use plain structs, we need
 * to use BOOST_DESCRIBE_STRUCT on them. This adds the structs the required reflection
 * data, so Boost.MySQL knows how to parse rows into them.
 */

#include <boost/mysql/error_with_diagnostics.hpp>
#include <boost/mysql/static_results.hpp>
#include <boost/mysql/tcp_ssl.hpp>

#include <boost/asio/io_context.hpp>
#include <boost/asio/ssl/context.hpp>
#include <boost/describe/class.hpp>
#include <boost/optional/optional.hpp>

#include <iostream>
#include <stdexcept>
#include <string>
#include <tuple>

// This header contains boilerplate code to parse the command line
// arguments into structs. Parsing the command line yields a cmdline_args,
// an alias for a boost::variant2::variant holding the command line
// arguments for any of the subcommands. We will use it via visit().
#include "parse_cmdline.hpp"

// Including any of the static interface headers brings this macro into
// scope if the static interface is supported.
#ifdef BOOST_MYSQL_CXX14

namespace mysql = boost::mysql;

namespace {

// An order retrieved by our system.
struct order
{
    // The unique database ID of the object.
    std::int64_t id;

    // The order status (draft, pending_payment, complete).
    std::string status;
};
BOOST_DESCRIBE_STRUCT(order, (), (id, status))

// A product, as listed in the store product catalog.
struct product
{
    // The unique database ID of the object.
    std::int64_t id;
    // A short name for the product. Can be used as a title.
    std::string short_name;

    // The product's description. This field can be NULL in the DB,
    // so we use boost::optional<T> for it. If you're using C++17 or higher,
    // you can use std::optional instead.
    boost::optional<std::string> descr;

    // The product's unit price, in cents of USD.
    std::int64_t price;
};
BOOST_DESCRIBE_STRUCT(product, (), (id, short_name, descr, price))

// An order with its line items. This record type is returned by JOINs
// from the orders and order_items tables. We use this type to retrieve both
// an order and its line items in a single operation.
// If the order contains no line items, the item_xxx fields are NULL.
struct order_with_items
{
    // The ID of the order
    std::int64_t order_id;

    // The status of the order
    std::string order_status;

    // The ID of the line item, or NULL if the order doesn't have any
    boost::optional<std::int64_t> item_id;

    // The number of units of this product that the user wants to buy,
    // or NULL if the order doesn't have line items
    boost::optional<std::int64_t> item_quantity;

    // The product's unit price, in cents of USD, or NULL if the order
    // doesn't have line items
    boost::optional<std::int64_t> item_unit_price;

    bool has_item() const
    {
        return item_id.has_value() && item_quantity.has_value() && item_unit_price.has_value();
    }
};
BOOST_DESCRIBE_STRUCT(
    order_with_items,
    (),
    (order_id, order_status, item_id, item_quantity, item_unit_price)
);

// An empty row type. This can be used to describe empty resultsets,
// like the ones returned by INSERT or CALL.
using empty = std::tuple<>;

// This visitor executes a sub-command and prints the results to stdout.
struct visitor
{
    mysql::tcp_ssl_connection& conn;

    static void print_order(const order& ord)
    {
        std::cout << "Order: id=" << ord.id << ", status=" << ord.status << '\n';
    }

    // Retrieves an order with its items. If the order exists, at least one record is returned.
    // If the order has line items, a record per item is returned. If the order has no items,
    // a single record is returned, and it will have its item_xxx fields set to NULL.
    mysql::static_results<order_with_items> get_order_with_items(std::int64_t order_id) const
    {
        mysql::statement stmt = conn.prepare_statement(R"%(
            SELECT
                ord.id AS order_id,
                ord.status AS order_status,
                item.id AS item_id,
                item.quantity AS item_quantity,
                prod.price AS item_unit_price
            FROM orders ord
            LEFT JOIN order_items item ON ord.id = item.order_id
            LEFT JOIN products prod ON item.product_id = prod.id
            WHERE ord.id = ?
        )%");

        mysql::static_results<order_with_items> result;
        conn.execute(stmt.bind(order_id), result);
        return result;
    }

    // Prints an order with its line items to stdout
    static void print_order_with_items(boost::span<const order_with_items> ord_items)
    {
        assert(!ord_items.empty());

        // Print the order
        std::cout << "Order: id=" << ord_items[0].order_id << ", status=" << ord_items[0].order_status
                  << '\n';

        // Print the items
        if (!ord_items[0].has_item())
        {
            std::cout << "No line items\n";
        }
        else
        {
            for (const auto& item : ord_items)
            {
                std::cout << "  Line item: id=" << *item.item_id << ", quantity=" << *item.item_quantity
                          << ", unit_price=" << *item.item_unit_price / 100.0 << "$\n";
            }
        }
    }

    // get-products <search-term>: full text search of the products table.
    // use this command to search the store for available products
    void operator()(const get_products_args& args) const
    {
        // Our SQL contains a user-supplied paremeter (the search term),
        // so we will be using a prepared statement
        mysql::statement stmt = conn.prepare_statement(R"%(
            SELECT id, short_name, descr, price
            FROM products
            WHERE MATCH(short_name, descr) AGAINST(?)
            LIMIT 5
        )%");

        // The product struct describes the shape of the rows that
        // we expect the server to send.
        mysql::static_results<product> products;
        conn.execute(stmt.bind(args.search), products);

        // Print the results to stdout
        std::cout << "Your search returned the following products:\n";
        for (const product& prod : products.rows())
        {
            std::cout << "* ID: " << prod.id << '\n'
                      << "  Short name: " << prod.short_name << '\n'
                      << "  Description: " << (prod.descr ? *prod.descr : "") << '\n'
                      << "  Price: " << prod.price / 100.0 << "$" << std::endl;
        }
        std::cout << std::endl;
    }

    // create-order: creates a new order. Orders are always created empty. This command
    // requires no arguments
    void operator()(const create_order_args&) const
    {
        // Since this is an INSERT, we don't expect any row to be returned.
        // empty is an alias for std::tuple<>, which tells static_results to expect
        // an empty resultset.
        mysql::static_results<empty> result;
        conn.execute("INSERT INTO orders VALUES ()", result);

        // We can use static_results::last_insert_id() to retrieve the ID of the newly
        // created object. last_insert_id() returns always a uint64_t. Our schema uses
        // plain INTs for the id field, so this cast is safe.
        order ord{static_cast<std::int64_t>(result.last_insert_id()), "draft"};
        print_order(ord);
    }

    // get-order <order-id>: retrieves order details
    void operator()(const get_order_args& args) const
    {
        // Retrieve the order with its items
        mysql::static_results<order_with_items> result = get_order_with_items(args.order_id);

        // If we didn't find any order, issue an error
        if (result.rows().empty())
        {
            throw std::runtime_error("Can't find order with id=" + std::to_string(args.order_id));
        }

        // Print the order to stdout
        std::cout << "Retrieved order\n";
        print_order_with_items(result.rows());
    }

    // get-orders: lists all orders. Orders are listed without their line items.
    void operator()(const get_orders_args&) const
    {
        // Since this query doesn't have parameters, we don't need a prepared statement,
        // and we can use a text query instead.
        mysql::static_results<order> result;
        conn.execute("SELECT id, `status` FROM orders", result);

        // Print the results to stdout
        if (result.rows().empty())
        {
            std::cout << "No orders found" << std::endl;
        }
        else
        {
            for (const order& ord : result.rows())
            {
                print_order(ord);
            }
        }
    }

    // add-line-item <order-id> <product-id> <quantity>: adds a line item to a given order
    void operator()(const add_line_item_args& args) const
    {
        // We will need to run several statements atomically, so we start a transaction.
        mysql::static_results<empty> empty_results;
        conn.execute("START TRANSACTION", empty_results);

        // To add a line item, we require the order to be in a draft status. Get the order to check this fact.
        mysql::statement stmt = conn.prepare_statement("SELECT id, `status` FROM orders WHERE id = ?");
        mysql::static_results<order> orders;
        conn.execute(stmt.bind(args.order_id), orders);
        if (orders.rows().empty())
        {
            // There is no such order
            throw std::runtime_error("Order with id=" + std::to_string(args.order_id) + " not found");
        }
        else if (orders.rows()[0].status != "draft")
        {
            // The order is no longer editable
            throw std::runtime_error("Order with id=" + std::to_string(args.order_id) + " is not editable");
        }

        // Insert the new line item. If the given product does not exist, the INSERT will fail
        // because of product_id's FOREIGN KEY constraint.
        stmt = conn.prepare_statement(
            "INSERT INTO order_items (order_id, product_id, quantity) VALUES (?, ?, ?)"
        );
        conn.execute(stmt.bind(args.order_id, args.product_id, args.quantity), empty_results);

        // We can use static_results::last_insert_id to get the ID of the new line item.
        auto new_line_item_id = empty_results.last_insert_id();

        // Retrieve the full order details
        mysql::static_results<order_with_items> order_results = get_order_with_items(args.order_id);

        // We're done - commit the transaction
        conn.execute("COMMIT", empty_results);

        // Print the results to stdout
        std::cout << "Created line item: id=" << new_line_item_id << "\n";
        print_order_with_items(order_results.rows());
    }

    // remove-line-item <line-item-id>: removes an item from an order
    void operator()(const remove_line_item_args& args) const
    {
        // We will need to run several statements atomically, so we start a transaction.
        mysql::static_results<empty> empty_results;
        conn.execute("START TRANSACTION", empty_results);

        // To remove a line item, we require the order to be in a draft status. Get the order to check this
        // fact.
        mysql::static_results<order> orders;
        auto stmt = conn.prepare_statement(R"%(
            SELECT orders.id, orders.`status`
            FROM orders
            JOIN order_items items ON (orders.id = items.order_id)
            WHERE items.id = ?
        )%");
        conn.execute(stmt.bind(args.line_item_id), orders);
        if (orders.rows().empty())
        {
            // The query hasn't matched any row - the supplied line item ID is not valid
            throw std::runtime_error(
                "The order item with id=" + std::to_string(args.line_item_id) + " does not exist"
            );
        }
        const order& ord = orders.rows()[0];
        if (ord.status != "draft")
        {
            // The order is no longer editable
            throw std::runtime_error("The order is not in an editable state");
        }

        // Remove the line item
        stmt = conn.prepare_statement("DELETE FROM order_items WHERE id = ?");
        conn.execute(stmt.bind(args.line_item_id), empty_results);

        // Retrieve the full order details
        mysql::static_results<order_with_items> order_results = get_order_with_items(ord.id);

        // We're done - commit the transaction
        conn.execute("COMMIT", empty_results);

        // Print results to stdout
        std::cout << "Removed line item from order\n";
        print_order_with_items(order_results.rows());
    }

    // checkout-order <order-id>: marks an order as ready for checkout
    void operator()(const checkout_order_args& args) const
    {
        // We will need to run several statements atomically, so we start a transaction.
        mysql::static_results<empty> empty_results;
        conn.execute("START TRANSACTION", empty_results);

        // To checkout an order, we require it to be in a draft status. Check this fact.
        mysql::statement stmt = conn.prepare_statement("SELECT id, `status` FROM orders WHERE id = ?");
        mysql::static_results<order> orders;
        conn.execute(stmt.bind(args.order_id), orders);
        if (orders.rows().empty())
        {
            // No order matched
            throw std::runtime_error("Order with id=" + std::to_string(args.order_id) + " not found");
        }
        else if (orders.rows()[0].status != "draft")
        {
            // The order is no longer editable
            throw std::runtime_error(
                "Order with id=" + std::to_string(args.order_id) + " cannot be checked out"
            );
        }

        // Update the order status
        stmt = conn.prepare_statement("UPDATE orders SET `status` = 'pending_payment' WHERE id = ?");
        conn.execute(stmt.bind(args.order_id), empty_results);

        // Calculate the total amount to pay. SUM() returns a DECIMAL, which has a bigger
        // range than integers. DECIMAL is represented in C++ as a string. We use CAST to obtain
        // an uint64_t. If the CAST overflows, the max value for uint64_t will be returned.
        // We will be limiting our orders to USD 1bn, so overflow will be detected.
        stmt = conn.prepare_statement(R"%(
            SELECT CAST(
                IFNULL(SUM(prod.price * item.quantity), 0)
                AS UNSIGNED
            )
            FROM order_items item
            JOIN products prod ON item.product_id = prod.id
            WHERE item.order_id = ?;
        )%");
        mysql::static_results<std::tuple<std::uint64_t>> amount_results;
        conn.execute(stmt.bind(args.order_id), amount_results);
        std::uint64_t total_amount = std::get<0>(amount_results.rows()[0]);

        // Verify that the total amount meets our criteria
        if (total_amount == 0)
        {
            throw std::runtime_error("The order doesn't have any line item");
        }
        else if (total_amount > 1000 * 1000 * 100)
        {
            throw std::runtime_error("Order amount of " + std::to_string(total_amount) + " exceeds limit");
        }

        // Retrieve the full order details
        mysql::static_results<order_with_items> order_results = get_order_with_items(args.order_id);

        // We're done - commit the transaction
        conn.execute("COMMIT", empty_results);

        // Print the results to stdout
        std::cout << "Checked out order. The total amount to pay is: " << total_amount / 100.0 << "$\n";
        print_order_with_items(order_results.rows());
    }

    // complete-order <order-id>: marks an order as completed
    void operator()(const complete_order_args& args) const
    {
        // We will need to run several statements atomically, so we start a transaction.
        mysql::static_results<empty> empty_results;
        conn.execute("START TRANSACTION", empty_results);

        // To complete an order, we require it to be in a pending_payment status. Check this fact.
        auto stmt = conn.prepare_statement("SELECT id, `status` FROM orders WHERE id = ?");
        mysql::static_results<order> orders;
        conn.execute(stmt.bind(args.order_id), orders);
        if (orders.rows().empty())
        {
            // Order not found
            throw std::runtime_error("Order with id=" + std::to_string(args.order_id) + " not found");
        }
        else if (orders.rows()[0].status != "pending_payment")
        {
            throw std::runtime_error(
                "Order with id=" + std::to_string(args.order_id) + " is not in pending_payment status"
            );
        }

        // Update status
        stmt = conn.prepare_statement("UPDATE orders SET `status` = 'complete' WHERE id = ?");
        conn.execute(stmt.bind(args.order_id), empty_results);

        // Retrieve the full order details
        mysql::static_results<order_with_items> order_results = get_order_with_items(args.order_id);

        // We're done - commit the transaction
        conn.execute("COMMIT", empty_results);

        // Print the results to stdout
        std::cout << "Completed order\n";
        print_order_with_items(order_results.rows());
    }
};

void main_impl(int argc, char** argv)
{
    // Parse command line arguments
    auto args = parse_cmdline_args(argc, argv);

    // I/O context and connection. We use SSL because MySQL 8+ default settings require it.
    boost::asio::io_context ctx;
    boost::asio::ssl::context ssl_ctx(boost::asio::ssl::context::tls_client);
    mysql::tcp_ssl_connection conn(ctx, ssl_ctx);

    // Resolver for hostname resolution
    boost::asio::ip::tcp::resolver resolver(ctx.get_executor());

    // Connection params
    mysql::handshake_params params(
        args.username,                  // username
        args.password,                  // password
        "boost_mysql_order_management"  // database to use
    );

    // Hostname resolution
    auto endpoints = resolver.resolve(args.host, mysql::default_port_string);

    // TCP and MySQL level connect
    conn.connect(*endpoints.begin(), params);

    // Execute the command
    boost::variant2::visit(visitor{conn}, args.cmd);

    // Close the connection
    conn.close();
}

}  // namespace

int main(int argc, char** argv)
{
    try
    {
        main_impl(argc, argv);
    }
    catch (const mysql::error_with_diagnostics& err)
    {
        // Some errors include additional diagnostics, like server-provided error messages.
        // Security note: diagnostics::server_message may contain user-supplied values (e.g. the
        // field value that caused the error) and is encoded using to the connection's encoding
        // (UTF-8 by default). Treat is as untrusted input.
        std::cerr << "Error: " << err.what() << ", error code: " << err.code() << '\n'
                  << "Server diagnostics: " << err.get_diagnostics().server_message() << std::endl;
        return 1;
    }
    catch (const std::exception& err)
    {
        std::cerr << "Error: " << err.what() << std::endl;
        return 1;
    }
}

#else

int main()
{
    std::cout << "Sorry, your compiler doesn't have the required capabilities to run this example"
              << std::endl;
}

#endif

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