Laravel pivot Eloquent – Attach vs. SyncWithoutDetaching

attach() will always add a new record to the pivot table, whereas syncWithoutDetaching() will only add a new record if one does not exist.

$order->items()->attach(1);
$order->items()->attach(1);

// $order->items()->count() === 2

$order2->items()->syncWithoutDetaching(1);
$order2->items()->syncWithoutDetaching(1);

// $order2->items()->count() === 1

sync(Array, false) is the same as syncWithoutDetaching():

public function syncWithoutDetaching($ids)
    {
        return $this->sync($ids, false);
    }

In short, by default sync() detaches.