php fatal error uncaught error using $this when not in object context

$r = $this->db->query($query);  // * вот на этой строчке

    $tree = array();

    while ($category = $this->db->fetchObject($r)) {

        // .... тут я работаю с категориями,

# Execute the query or queries array
public function query($q)
{
    if ($this->link) {
        $start = microtime(true);
        $this->res_id = mysqli_query($this->link,$q);
    }
    else {
        $this->error_msg = "Could not execute query to $this->db_name database, wrong database link";
        return 0;
    }
    if (!$this->res_id && $this->exception_on_error) {
        trigger_error("Could not execute query to $this->db_name database, wrong result id", E_USER_ERROR);
        return 0;
    }
    return $this->res_id;
}