Fatal error: Call to undefined function dbDelta()

I want to create a table during the activation of the plugin, so I used the code as follows:

class Database
{
    private $db_version = '1.0', $table_prefix;

    public function __construct()
    {
        global $wpdb;
        $this->table_prefix = $wpdb->prefix;
        register_activation_hook( PLUGIN_INDEX_FILE, array($this, 'dbSetup') );
    }

    public function dbSetup()
    {
        $countriesSQL = "CREATE TABLE $this->table_prefix . countries (
            id int(11) UNSIGNED NOT NULL,
            country_code varchar(2) NOT NULL DEFAULT '',
            country_name varchar(100) NOT NULL DEFAULT '',
            PRIMARY KEY  (id)
        ) $charset_collate;";
        dbDelta( $countriesSQL );
    }
}

But during the activation of plugin, it throws the fatal error as,

Fatal error: Call to undefined function dbDelta() in
wp-content/plugins/myPlugin/libraries/Database.class.php on line 26

In some forums, some are suggesting to use,
require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
before the function call.

It is working. But I just wanted to know, if it is this the right approach? How to use the dbDelta in the correct manner to make it work?

You answered your own question. You need to include the upgrade.php file which contains that function. Otherwise won’t be loaded on your plugin update and the function won’t exist.

require_once(ABSPATH . 'wp-admin/includes/upgrade.php');