Magento 1.9 – How to Display Only Subcategories in Layered Navigation

I am not sure how your homepage is configured, but by default you can't make it happen.

Try adding a Category block on your homepage, by adding

{{block type="catalog/product_list" category_id="8" template="catalog/product/list.phtml"}}

on you Homepage CMS's page content. Replace category_id with your category's id.

Cheers.

As you already know the real category filtering is taking place in app/code/core/Mage/Catalog/Model/Layer/Filter/Category.php, more specifically in _getItemsData(). Let us inspect that method.

<?php
            $categoty   = $this->getCategory();
            $categories = $categoty->getChildrenCategories();

            $this->getLayer()->getProductCollection()
                ->addCountToCategories($categories);

            $data = array();

            foreach ($categories as $category) {
                if ($category->getIsActive() && $category->getProductCount()) {
                    $data[] = array(
                        'label' => Mage::helper('core')->escapeHtml($category->getName()),
                        'value' => $category->getId(),
                        'count' => $category->getProductCount(),
                    );
                }
            }

As you can see, the method loads child categories of $categoty in layered navigation filter. That means we need to set this variable to a parent category in level 2 (Every main categories lies in level 2), according to the current category that we stands.

We can achieve this in many ways. In my method, I dig into the codes further and find the exact position where the category is set for layered navigation.

It resides here app/code/core/Mage/Catalog/Model/Layer.php in getCurrentCategory(). The method look like this.

<?php
 public function getCurrentCategory()
{
    $category = $this->getData('current_category');
    if (is_null($category)) {
        if ($category = Mage::registry('current_category')) {
            $this->setData('current_category', $category);
        }
        else {
            $category = Mage::getModel('catalog/category')->load($this->getCurrentStore()->getRootCategoryId());
            $this->setData('current_category', $category);
        }
    }

    return $category;
}

In short, it loads the current category to the variable $categoty (variable that I mentioned in _getItemsData()) if it set, otherwise loads the root-category to variable categoty. Replace this method with this code

<?php
/**
 * Retrieve current category model
 * If no category found in registry, the root will be taken
 *
 * @return Mage_Catalog_Model_Category
 */
public function getCurrentCategory()
{
    $category = $this->getData('current_category');

    if (is_null($category)) {
        $category = Mage::registry('current_category');
        if ($category) {
            /*
             * Check whether the current category is in level 2.
             * If yes set it as current category.
             * Else check for any parent categories with level 2
             */
            if($category->getLevel() != 2) {
                //gets all parent categories
                $parentCategories = Mage::registry('current_category')->getParentCategories();
                /*
                 * If parent categories exist,search for parent categories in level 2.
                 * If exist set it as current category.
                 *else set root category as current category.
                */
                if(count($parentCategories) > 0)
                {
                    foreach ( $parentCategories as $parentCategory ) {
                        if($parentCategory->getLevel() == 2 ) {
                            $category = Mage::getModel('catalog/category')->load($parentCategory->getId());
                            $this->setData('current_category', $category);
                            break;
                        }
                    }
                }
                else {

                    $category = Mage::getModel('catalog/category')->load($this->getCurrentStore()->getRootCategoryId());
                    $this->setData('current_category', $category);
                }
            }
            else {

                $this->setData('current_category', $category);
            }
        }
        else {

            $category = Mage::getModel('catalog/category')->load($this->getCurrentStore()->getRootCategoryId());
            $this->setData('current_category', $category);
        }
    }

    return $category;
}

What we are doing here is, we get the parent category in level 2 from the current category and set it to $categoty, so that we can see all child categories in layered navigation.

As I already mentioned, you can achieve it in many ways. All you need to do is to set $categoty variable with parent category in level 2. Above I have altered the root files. It is not a good practice. You have to create a module that rewrites the model file app/code/core/Mage/Catalog/Model/Layer.php and then overwrite the method getCurrentCategory().

Hope it helps