public function participantsString($userId=null, $columns=['first_name'])
    {
        $selectString = $this->createSelectString($columns);

        $participantNames = $this->getConnection()->table($this->getUsersTable())
            ->join('participants', $this->getUsersTable() . '.id', '=', 'participants.user_id')
            ->where('participants.thread_id', $this->id)
            ->select($this->getConnection()->raw($selectString));

        if ($userId !== null) {
            $participantNames->where($this->getUsersTable() . '.id', '!=', $userId);
        }

        $userNames = $participantNames->lists($this->getUsersTable() . '.name');

        return implode(', ', $userNames);
    }