Magento: Extended Version of the related products in the community edition / Get a product collection and filter by a multiselect field with another multiselect field. sto_artsc3_line is the multiselect field, type varchar. FIND_IN_SET() handles the magic in MySQL. If anybody knows a better solution, post it here :-)

Related.php

<?php
  /**
     * find all products by line and gender
     *
     * @param $currentProduct Mage_Catalog_Model_Product
     *
     * @return Mage_Catalog_Model_Resource_Product_Collection
     */
    protected function _getProductByLineCollection(Mage_Catalog_Model_Product $currentProduct)
    {
        $stoArtsc3Line = $currentProduct->getStoArtsc3Line();

        if (empty($stoArtsc3Line)) {
            return null;
        }

        $productLineCollection = Mage::getResourceSingleton('catalog/product_collection');
        /* @var $productLineCollection Mage_Catalog_Model_Resource_Product_Collection */

        $productLineCollection
            ->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes())
            ->addMinimalPrice()
            ->addWebsiteFilter()
            ->addFinalPrice()
            ->addTaxPercents()
            ->addUrlRewrite(0);

        Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($productLineCollection);
        Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($productLineCollection);

        $productLineCollection
            ->addAttributeToFilter('sku', array('neq' => $currentProduct->getSku()))
            ->addAttributeToFilter('sto_artsc3_line', array('neq' => ''))
            ->addAttributeToFilter('sto_artsd1_gender', array('eq' => $currentProduct->getStoArtsd1Gender()));

        $select = $productLineCollection->getSelect();
        /* @var $select Varien_Db_Select */

        // could be comma string getStoArtsc3Line
        $lineNumbers = Mage::helper('xxxx')->trimExplode(',', $stoArtsc3Line);

        $where = array();
        foreach ($lineNumbers as $value) {
            /* http://dev.mysql.com/doc/refman/5.1/de/string-functions.html */
            $value   = (int)$value;
            $where[] = '(FIND_IN_SET(\'' . $value . '\',at_sto_artsc3_line.value)>0)';
        }

        $select->where(implode(' or ', $where));

        return $productLineCollection;
    }

at_sto_artsc3_line has been generated from the addAttributeToFilter() method