Event::whereHas('participants', function ($query) { $query->where('IDUser', '=', 1); })->get();
$account = App\Account::find(10); $user->account()->associate($account); $user->save();
Event::with(["owner", "participants" => function($q) use($someId){ $q->where('participants.IdUser', '=', 1); //$q->where('some other field', $someId); }])