Something I don't understand in filterBy() and findBy()

I am having trouble using filterBy and findBy on users and pages fields.

I assumed they would work like this (owner is a users field):

$playlists = $children->filterBy('owner', kirby()->user());

but that doesn’t work.

$playlists = $children->filterBy('owner', kirby()->user()->username());
$playlists = $children->filterBy('owner', kirby()->user()->id());

don’t work either.

The only way I can get it to work is this:

$playlists = $children->filterBy('owner', '- '.kirby()->user()->id());

The same goes for the pages field.
I assume I am doing something wrong because having to add a string to my userid feels icky.

What would be the correct way to use filterBy and findBy?

Aah, thank you very much.