// in a single line here
$values = Cart::where('session_id', $id)->update(['data'=>$data]);
// If there's a flight from Oakland to San Diego, set the price to $99.
// If no matching model exists, create one.
$flight = AppFlight::updateOrCreate(
['departure' => 'Oakland', 'destination' => 'San Diego'],
['price' => 99]
);