mysqli sum()

[techno]

Perche non mi da errore su num_rows?

class database{

    public $host="localhost";
    public $user="root";
    public $password="sim31gia08";
    public $db="concadoro";
    public $dbcomm;
    public $result;
    
    function connect(){
        $this->dbcomm=mysqli_connect($this->host,$this->user,$this->password) or die (mysqli_error());
        mysqli_select_db($this->dbcomm,$this->db) or die (mysqli_error());
    }
    
    function query($query){
        $this->result=mysqli_query($this->dbcomm,$query);
        return $this->result;
    }
    
    function disconnect(){
        mysqli_close($this->dbcomm);
    }
}

$database=new database;
$database->connect();

$sql="UPDATE cart SET Quantita=Quantita + '$quantita' WHERE IDShop='$idshop' AND IDarticolo='$idarticolo'";
$query=$database->query($sql);
if($query->num_rows==0){
    $sql="INSERT INTO cart (IDShop, IDarticolo, Quantita) VALUES ('$idshop','$idarticolo','$quantita')";
    if(!$database->query($sql)){
        $response="Errore nell'aggiungere l'articolo al carrello, riprova.";
        $error=true;
    }else{
        $response="Articolo aggiunto al carrello.";
    }
}else{
    $response="É stata aggiunta la quantita a questo articolo.";
} 

Mi da errore trying to property on non-object...