Magento 1.9 fetch specific cateogry on top level menu in header

Question

I have added custom attribute for Categories section .I want to fetch specific categories only based on custom attribute value in header . I want to modify this code :

$this->getChildHtml('topMenu') 

$_menu = $this->getHtml('level-top')

I found this method getHtml() , but I am not understanding where to alter query to get specific categories.

Answer 1

TL;DR

you need to override method _getDefaultCollection in Mage_Catalog_Model_Resource_Category_Tree :

protected function _getDefaultCollection($sorted = false)
{
    $this->_joinUrlRewriteIntoCollection = true;
    $collection = Mage::getModel('catalog/category')->getCollection();
    /** @var $collection Mage_Catalog_Model_Resource_Category_Collection */
    // your filters here
    $collection = $collection->addAttributeToFilter('attribute_code', 'attribute_value');

 ...
 }

More detailed description:

The category tree rendering on frontend starts in app/code/core/Mage/Page/Block/Html/Topmenu.php

when event page_block_html_topmenu_gethtml_before is fired:

public function getHtml($outermostClass = '', $childrenWrapClass = '')
{
    Mage::dispatchEvent('page_block_html_topmenu_gethtml_before', array(
        'menu' => $this->_menu,
        'block' => $this
    ));
...

This event is listened by observer in Mage_Catalog_Block_Category::addCatalogToTopmenuItems which calls method getStoreCategories of catalog/category helper:

public function addCatalogToTopmenuItems(Varien_Event_Observer $observer)
{
    ...
    $this->_addCategoriesToMenu(
        Mage::helper('catalog/category')->getStoreCategories(), $observer->getMenu(), $block
    );
}

Which eventually leads to :

public function getStoreCategories($sorted=false, $asCollection=false, $toLoad=true)
{
    ...
    $storeCategories = $category->getCategories($parent, $recursionLevel, $sorted, $asCollection, $toLoad);
    ...
}

Where getCategories method is lead to Mage_Catalog_Model_Resource_Category_Tree resource model method.