function create_line_item($purchase_id, $item_id, $price)
{
    global $wpdb;
    $line_item_table = get_line_items_table_name();
    $wpdb->insert($line_item_table, array('purchase_id' => $purchase_id, 'item_id' => $item_id, 'price' => $price));
}
function build_line_items_sql()
{
    $line_items_table = get_line_items_table_name();
    $items_table = get_items_table_name();
    $purchases_table = get_purchases_table_name();
    $sql = "CREATE TABLE {$line_items_table} (\n    `purchase_id` int(10) NOT NULL,\n    `item_id` int(10) NOT NULL,\n    `price` decimal(10, 2),\n    PRIMARY KEY (`purchase_id`, `item_id`),\n    FOREIGN KEY (`purchase_id`) REFERENCES {$purchases_table}(`id`),\n    FOREIGN KEY (`item_id`) REFERENCES {$items_table}(`id`)\n    );";
    return $sql;
}