public function findUserIdsWithOnlyOneGroup()
    {
        $query = $this->find();
        return $query
            ->select([
                'user_id',
                'group_count' => $query->func()->count('*')
            ])
            ->group('user_id')
            ->having([
                'group_count = 1'
            ])
            ->extract('user_id')
            ->toArray();
    }