Magento – “random” order clause added to the collection

For a module I load a custom product collection where I select products based on IDs in an array, I also try and sort the collection by the same order as the IDs have in the array().

To achieve the sorting I use Zend's getSelect->order() to add a custom "ORDER BY" to the SQL statement.

$collection = Mage::getModel('catalog/product')->getCollection()
            ->addAttributeToSelect('*')
            ->addAttributeToFilter('entity_id', $productIds)
            ->addOrder('entity_id');

$sortIds = array_map('intval', $productIds);
$collection->getSelect()
           ->order("find_in_set(e.entity_id,'".implode(',', $sortIds)."')");

This will generate a SQL error: "SQLSTATE[42S22]: Column not found: 1054 Unknown column 'cat_index_position' in 'order clause'", since I don't have the "cat_index_position" in my product flat table.

This error is because Magento adds an extra ORDER BY clause: 'cat_index_position' ASC after the one I add with getSelect()->order()

If I don't add the getSelect()->order() no extra ORDER BY is added and no SQL error is generated, however the sort order is obviously wrong.

I can, very ugly, hack this by commenting out the rest of the SQL statement like this $collection->getSelect()->order("find_in_set(e.entity_id,'".implode(',',$ids)."');--"); but I really really don't want to do that.

I need help tracking down the source of that extra ORDER BY clause or any other way of solving it.

UPDATE

If I do $collection->getSelect()->__toString() after I've added my custom order I get:

SELECT 1 AS status, e.entity_id, e.type_id, e.attribute_set_id
FROM catalog_product_flat_1 AS e
WHERE (((e.entity_id = '8534') OR (e.entity_id = '4184') OR (e.entity_id = '4185') OR (e.entity_id =
'4183') OR (e.entity_id = '13314') OR (e.entity_id = '5069') OR
(e.entity_id = '11217') OR (e.entity_id = '4355') OR (e.entity_id =
'4354') OR (e.entity_id = '4353') OR (e.entity_id = '4352') OR
(e.entity_id = '4351') OR (e.entity_id = '4350') OR (e.entity_id =
'4349') OR (e.entity_id = '4348') OR (e.entity_id = '5203'))) ORDER BY
find_in_set(e.entity_id,'8534,4184,4185,4183,13314,5069,11217,4355,4354,4353,4352,4351,4350,4349,4348,5203')
ASC

And as I said previously, now Magento adds an extra ORDER BY, because the error I get say I have this SQL statement:

SELECT 1 AS status, e.entity_id, e.type_id, e.attribute_set_id
FROM catalog_product_flat_1 AS e
WHERE (((e.entity_id = '8534') OR (e.entity_id = '4184') OR (e.entity_id = '4185') OR (e.entity_id =
'4183') OR (e.entity_id = '13314') OR (e.entity_id = '5069') OR
(e.entity_id = '11217') OR (e.entity_id = '4355') OR (e.entity_id =
'4354') OR (e.entity_id = '4353') OR (e.entity_id = '4352') OR
(e.entity_id = '4351') OR (e.entity_id = '4350') OR (e.entity_id =
'4349') OR (e.entity_id = '4348') OR (e.entity_id = '5203'))) ORDER BY
find_in_set(e.entity_id,'8534,4184,4185,4183,13314,5069,11217,4355,4354,4353,4352,4351,4350,4349,4348,5203')
ASC, cat_index_position ASC, e.entity_id ASC LIMIT 16