Populate multiple select field in Laravel

I try to populate a multiple select field in laravel like this:

The option are not select in the dropdown. I then tried this:

$partenaire->maisons->pluck('id')

and it returns an array: [1,2,3] (example)

I then tried to put the array manually in the field like this:

The previous line returns the select field with the select iptions.

What is wrong then ?

via Lucien Dubois