SQL slightly complex question

Hi,

I have quite a rookie question about nette DB

I want to do the following query

SELECT
	deposit_materials_id, SUM(amount)
FROM
	table
WHERE
	users_id = ?
GROUP BY
	deposit_materials_id

How do I forge this query in nette DB?

I tried this:

<?php
		return $this->database->table(self::TABLE_DEPOSIT_USERS)
				->where('users_id', (int)$id)
				->group('deposit_materials_id')
				->sum('amount');
?>

but the final generated query is missing the group portion and returning only one result:

SELECT SUM(`amount`)
FROM `deposit_materials_users`
WHERE (`users_id` = 70)

I don't want to make a direct query because i want to keep the ref functionality to other tables

Thanks for your suggestions