Call to a member function select() on a non-object - joomla

Question

I'm trying to retrieve some info using ajax. however when I call the select() method the system returns a fatal error. what is the reason for that?
here's my code:
define( '_JEXEC', 1 );
define( 'DS', DIRECTORY_SEPARATOR );
define( 'JPATH_BASE', realpath(dirname(__FILE__).DS.'..' ));
require_once ( JPATH_BASE .DS.'includes'.DS.'defines.php' );
require_once ( JPATH_BASE .DS.'includes'.DS.'framework.php' );
$db = JFactory::getDbo();
$query = $db->getQuery(true);
$query->select($db->quoteName(array('id', 'product_id')));
$query->from($db->quoteName('sample_table'));
// ->where('application_id = 11');
$db->setQuery($query);
$results = $db->loadObjectList();
I have used this code several times on other versions and it worked. I think it might be related to the version..
thanks in advance.

Answer 1

As per your code , I think you are trying to use Joomla function in external function. The error indicates that you are using the factory object without including the factory class. Try include all these files :
define('_JEXEC', 1);
define('DS', DIRECTORY_SEPARATOR);
define('JPATH_BASE', $_SERVER['DOCUMENT_ROOT'] . DS . '');
require_once (JPATH_BASE . DS . 'includes' . DS . 'defines.php');
require_once (JPATH_BASE . DS . 'includes' . DS . 'framework.php');
require_once (JPATH_BASE . DS . 'libraries' . DS . 'joomla' . DS . 'factory.php');
$db = JFactory::getDbo();
$query = $db->getQuery(true);
$query->select($db->quoteName(array('id', 'product_id')));
$query->from($db->quoteName('sample_table'));
// ->where('application_id = 11');
$db->setQuery($query);
$results = $db->loadObjectList();

Answer 2

Is that all of your code? It looks to me like you have not created a database connection. You need something like
jimport('joomla.database.database');
// System configuration.
$config = JFactory::getConfig();
// Note, this will throw an exception if there is an error
// Creating the database connection.
$this->dbo = JDatabase::getInstance(
array(
'driver' => $config->get('dbtype'),
'host' => $config->get('host'),
'user' => $config->get('user'),
'password' => $config->get('password'),
'database' => $config->get('db'),
'prefix' => $config->get('dbprefix'),
)
);
Overall with a stand alone application it takes a few more minutes the first time but I think you are better off using the API and putting it in a constructor. Here's an example https://github.com/elinw/AssetFix/blob/j3/assetfix.php#L65