Laravel how to override object property?

Question

Laravel

There is this simple code:

$offer = Offer::with([
    'countries' => function ($query) {
        $query->select('offer_countries.id', 'offer_countries.abbr');
    }
])->findOrFail($id);

Let's say I want to make an array from the $offer->countries collection only from the id-shniks of each country and override this property. How to do it?
The variant below does not work, $offer->countries remains unchanged during the output:

$offer->countries = $offer->countries->map(function($country) {
    return $country->id;
});
return $offer;

Answer 1

dump() before and after map?
Is it really unchanged?
maybe there is something wrong after the return?
map can be changed to:
$offer->countries->pluck('id');