Subselect as main_table on magento collection

Question

I'm trying to do a query with subselect in magento collection.

Example

The original query:

$collection = Mage::getModel('salesrule/rule')
    ->getCollection()
    ->addFieldToFilter('rule_type', 2)
    ->addWebsiteFilter($customer->getWebsiteId())
;

$collection->getSelect()
    ->joinInner(
        array('special' => $collection->getTable('company_salesrule/special')),
        'special.salesrule_id=main_table.rule_id'
    )
    ->where('special.from where('special.to > ?', $weight)
    //->order('main_table.sort_order ASC')
    ->group('special.group')
;

With the result;

SELECT 
    `main_table` . *, `rule_coupons`.`code`, `special` . *
FROM
    `salesrule` AS `main_table`
        LEFT JOIN
    `salesrule_coupon` AS `rule_coupons` ON main_table.rule_id = rule_coupons.rule_id
        AND rule_coupons.is_primary = 1
        INNER JOIN
    `company_salesrule_special` AS `special` ON special.salesrule_id = main_table.rule_id
WHERE
    (rule_type = '2')
        AND (EXISTS( SELECT 
            1
        FROM
            `salesrule_website` AS `website`
        WHERE
            (website.website_id IN ('6'))
                AND (main_table.rule_id = website.rule_id)))
        AND (special.from  100)
GROUP BY special.group

But, I need this result (Line 4):

SELECT 
    `main_table` . *, `rule_coupons`.`code`, `special` . *
FROM
    (SELECT * FROM salesrule ORDER BY sort_order ASC) AS `main_table`
        LEFT JOIN
    `salesrule_coupon` AS `rule_coupons` ON main_table.rule_id = rule_coupons.rule_id
        AND rule_coupons.is_primary = 1
        INNER JOIN
    `company_salesrule_special` AS `special` ON special.salesrule_id = main_table.rule_id
WHERE
    (rule_type = '2')
        AND (EXISTS( SELECT 
            1
        FROM
            `salesrule_website` AS `website`
        WHERE
            (website.website_id IN ('6'))
                AND (main_table.rule_id = website.rule_id)))
        AND (special.from  100)
GROUP BY special.group

Answer 1

You can try something like:

$subQuery = new Zend_Db_Select();

...

$collection->getSelect()->from($subQuery);