Erro MySQL

Olá.

Gostaria de pedir ajuda a comunidade iMaster. Estou com um erro persistente no meu código PHP que não consigo resolver.

Que é o seguinte: Fatal error: Call to a member function select() on a non-object in C:\Program Files (x86)\VertrigoServ\www\sims\includes\class-query.php on line 53

Meu código de class.query é:

<?php
	require_once('./config.php');

	if ( !class_exists('QUERY') ) {
 class QUERY {
 public function load_user_object($usuario_id) {
 global $db;

 $table = 'usuarios';

 $query = "
 SELECT * FROM $table
 WHERE usuario_id = $usuario_id
 ";

 $obj = $db->select($query);

 if ( !$obj ) {
 return "No user found";
 }

 return $obj[0];
 }

 public function load_all_user_objects() {
 global $db;

 $table = 'usuarios';

 $query = "
 SELECT * FROM $table
 ";

 $obj = $db->select($query);

 if ( !$obj ) {
 return "No user found";
 }

 return $obj;
 }

 public function get_friends($usuario_id) {
 global $db;

 $table = 's_friends';

 $query = "
 SELECT ID, friend_id FROM $table
 WHERE usuario_id = '$usuario_id'
 ";

 $friends = $db->select($query);

 foreach ( $friends as $friend ) {
 $friend_ids[] = $friend->friend_id;
 }

 return $friend_ids;

 }

 public function get_status_objects($usuario_id) {
 global $db;

 $table = 's_status';

 $friend_ids = $this->get_friends($usuario_id);

 if ( !empty ( $friend_ids ) ) {
 array_push($friend_ids, $usuario_id);
 } else {
 $friend_ids = array($usuario_id);
 }

 $accepted_ids = implode(', ', $friend_ids);

 $query = "
 SELECT * FROM $table
 WHERE usuario_id IN ($accepted_ids)
 ORDER BY status_time DESC
 ";

 $status_objects = $db->select($query);

 return $status_objects;
 }

 public function get_message_objects($usuario_id) {
 global $db;

 $table = 's_messages';

 $query = "
 SELECT * FROM $table
 WHERE message_recipient_id = '$usuario_id'
 ";

 $messages = $db->select($query);

 return $messages;
 }

 public function do_user_directory() {
 $users = $this->load_all_user_objects();

 foreach ( $users as $user ) { ?>
 <div class="directory_item">
 <h3><a href="/social/profile-view.php?uid=<?php echo $user->ID; ?>"><?php echo $user->user_nicename; ?></a></h3>
 <p><?php echo $user->user_email; ?></p>
 </div>
 <?php
 }
 }

 public function do_friends_list($friends_array) {
 foreach ( $friends_array as $friend_id ) {
 $users[] = $this->load_user_object($friend_id);
 }

 foreach ( $users as $user ) { ?>
 <div class="directory_item">
 <h3><a href="/social/profile-view.php?uid=<?php echo $user->ID; ?>"><?php echo $user->user_nicename; ?></a></h3>
 <p><?php echo $user->user_email; ?></p>
 </div>
 <?php
 }
 }

 public function do_news_feed($usuario_id) {
 $status_objects = $this->get_status_objects($usuario_id);

 foreach ( $status_objects as $status ) {?>
 <div class="status_item">
 <?php $user = $this->load_user_object($status->usuario_id); ?>
 <h3><a href="/social/profile-view.php?uid=<?php echo $user->ID; ?>"><?php echo $user->user_nicename; ?></a></h3>
 <p><?php echo $status->status_content; ?></p>
 </div>
 <?php
 }
 }

 public function do_inbox($usuario_id) {
 $message_objects = $this->get_message_objects($usuario_id);

 foreach ( $message_objects as $message ) {?>
 <div class="status_item">
 <?php $user = $this->load_user_object($message->message_sender_id); ?>
 <h3>From: <a href="/social/profile-view.php?uid=<?php echo $user->ID; ?>"><?php echo $user->user_nicename; ?></a></h3>
 <p><?php echo $message->message_subject; ?></p>
 <p><?php echo $message->message_content; ?></p>
 </div>
 <?php
 }
 }
 }
	}

	$query = new QUERY;
?>

Desde já obrigado.