Subcategories in layered navigation filter

Question

I have a category with simple two-level structure like this:

Category #1
- Subcategory
- Subcategory
- ...
Category #2
- Subcategory
- Subcategory
- ...

Currently to filter by subcategories - you have to select top-level category first.

How to show subcategories of all top-level categories in layered navigation filter?

Note: Subcategories should by effected by other selected attribute filter.

Answer 1

While experimenting with Magento files I've found the answer to my question.

1. Copy

   app/code/core/Mage/Catalog/Model/Layer/Filter/Category.php

   to

   app/code/local/Mage/Catalog/Model/Layer/Filter/Category.php

2. Open copied file. And replace _getItemsData with code below:

   /**
    * Get data array for building category filter items
    *
    * @return array
    */
   protected function _getItemsData()
   {
     $key = $this->getLayer()->getStateKey().'_SUBCATEGORIES';
     $data = $this->getLayer()->getAggregator()->getCacheData($key);
   
     if ($data === null) {
       // Get root category
       $root_category = Mage::getModel('catalog/category')->load(2);
   
       // Get main categories
       $data = array();
       $main_categories = $root_category->getChildrenCategories();
       foreach ($main_categories as $main_category) {
         if (!$main_category->getIsActive()) continue; // Ommit inactive
         // Get sub categories to list
         $sub_categories = $main_category->getChildrenCategories();
   
         // Add count to subcategories
         $this->getLayer()->getProductCollection()
           ->addCountToCategories($sub_categories);
   
         foreach ($sub_categories as $sub_category) {
           // Ommit inactive and zero product count sub categories
           if ($sub_category->getIsActive() || !$sub_category->getProductCount()) continue;
   
           // Output subcategories
           $data[] = array(
             'label' => Mage::helper('core')->htmlEscape($sub_category->getName()),
             'value' => $sub_category->getId(),
             'count' => $sub_category->getProductCount(),
             'parent' => $main_category->getName(), // Store parent name to group in template
           );
         }
       }
   
       $tags = $this->getLayer()->getStateTags();
       $this->getLayer()->getAggregator()->saveCacheData($data, $key, $tags);
     }
     return $data;
   }
   

You might be interested in rewriting some other functions such as getResetValue, etc. I had to rewrite template to group subcategories by main categories.

Result (sorry cant post images directly):

Before: https://i.sstatic.net/skZpi.png

After: https://i.sstatic.net/QxPhq.png