Laravel where on relationship object

The correct syntax to do this on your relations is:

Event::whereHas('participants', function ($query) {
    return $query->where('IDUser', '=', 1);
})->get();

This will return Events where Participants have a user ID of 1. If the Participant doesn't have a user ID of 1, the Event will NOT be returned.

Read more at https://laravel.com/docs/5.8/eloquent-relationships#eager-loading

With multiple joins, use something like this code:

$userId = 44;
Event::with(["owner", "participants" => function($q) use($userId ){
    $q->where('participants.IdUser', '=', 1);
    //$q->where('some other field', $userId );
}])

for laravel 8.57+

Event::whereRelation('participants', 'IDUser', '=', 1)->get();

@Cermbo's answer is not related to this question. In that answer, Laravel will give you all Events if each Event has 'participants' with IdUser of 1.

But if you want to get all Events with all 'participants' provided that all 'participants' have a IdUser of 1, then you should do something like this :

Event::with(["participants" => function($q){
    $q->where('participants.IdUser', '=', 1);
}])

N.B:

In where use your table name, not Model name.