How to display categories and subcategories in layered navigation magento2?

Question

These are Categories List in admin:

If I click Shop then I want to display categories and sub-categories in layered navigation filter.

How can I achieve this functionality?

Answer 1

You need to overwrite some core models. This cannot be achieved using Magento UI.

The model you need to overwrite is here: Magento\CatalogSearch\Model\Layer\Filter\Category

Create a new module and add this to your di.xml file:

<preference for="Magento\CatalogSearch\Model\Layer\Filter\Category" type="Vendor\Module\Model\Layer\Filter\Category" />

Try this code in your model:

<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
 namespace Vendor\Module\Model\Layer\Filter;

  use Magento\Catalog\Model\Layer\Filter\AbstractFilter;
  use Magento\Catalog\Model\Layer\Filter\DataProvider\Category as     CategoryDataProvider;

 class Category extends AbstractFilter
 {
/**
 * @var \Magento\Framework\Escaper
 */
private $escaper;

/**
 * @var CategoryDataProvider
 */
private $dataProvider;
protected $_logger;
/**
 * @param \Magento\Catalog\Model\Layer\Filter\ItemFactory $filterItemFactory
 * @param \Magento\Store\Model\StoreManagerInterface $storeManager
 * @param \Magento\Catalog\Model\Layer $layer
 * @param \Magento\Catalog\Model\Layer\Filter\Item\DataBuilder $itemDataBuilder
 * @param \Magento\Catalog\Model\CategoryFactory $categoryFactory
 * @param \Magento\Framework\Escaper $escaper
 * @param CategoryManagerFactory $categoryManager
 * @param array $data
 */
public function __construct(   
    \Magento\Catalog\Model\Layer\Filter\ItemFactory $filterItemFactory,
    \Magento\Store\Model\StoreManagerInterface $storeManager,
    \Magento\Catalog\Model\Layer $layer,
    \Magento\Catalog\Model\Layer\Filter\Item\DataBuilder $itemDataBuilder,
    \Magento\Framework\Escaper $escaper,
    \Magento\Catalog\Model\Layer\Filter\DataProvider\CategoryFactory $categoryDataProviderFactory,
    \Psr\Log\LoggerInterface $logger, //log injection

    array $data = []
) {
    parent::__construct(
        $filterItemFactory,
        $storeManager,
        $layer,
        $itemDataBuilder,
        $data
    );
    $this->escaper = $escaper;
    $this->_requestVar = 'cat';
    $this->dataProvider = $categoryDataProviderFactory->create(['layer' => $this->getLayer()]);
    $this->_logger = $logger;
}

/**
 * Apply category filter to product collection
 *
 * @param   \Magento\Framework\App\RequestInterface $request
 * @return  $this
 */
public function apply(\Magento\Framework\App\RequestInterface $request)
{
    $categoryId = $request->getParam($this->_requestVar) ?: $request->getParam('id');
    if (empty($categoryId)) {
        return $this;
    }

    $this->dataProvider->setCategoryId($categoryId);

    $category = $this->dataProvider->getCategory();

    $this->getLayer()->getProductCollection()->addCategoryFilter($category);

    if ($request->getParam('id') != $category->getId() && $this->dataProvider->isValid()) {
        $this->getLayer()->getState()->addFilter($this->_createItem($category->getName(), $categoryId));
    }
    return $this;
}

/**
 * Get filter value for reset current filter state
 *
 * @return mixed|null
 */
public function getResetValue()
{
    return $this->dataProvider->getResetValue();
}

/**
 * Get filter name
 *
 * @return \Magento\Framework\Phrase
 */
public function getName()
{
    return __('Category');
}

/**
 * Get data array for building category filter items
 *
 * @return array
 */
protected function _getItemsData()
{
    $_objectManager = \Magento\Framework\App\ObjectManager::getInstance();
    $productCollection = $this->getLayer()->getProductCollection();

    $category = $this->dataProvider->getCategory();

    $optionsFacetedData = $productCollection->getFacetedData('category');
    $categories = $category->getChildrenCategories();
    $collectionSize = $productCollection->getSize();

    if ($category->getIsActive()) {
        foreach ($categories as $category) {
            if ($category->getIsActive()
                && isset($optionsFacetedData[$category->getId()])
                && $this->isOptionReducesResults($optionsFacetedData[$category->getId()]['count'], $collectionSize)
            ) {
                $this->itemDataBuilder->addItemData(
                    $this->escaper->escapeHtml($category->getName().'first-tier'.$category->getParentId()),
                    $category->getId(),
                    $optionsFacetedData[$category->getId()]['count']
                );
            }
                $cat = $_objectManager->create('Magento\Catalog\Model\Category')->load($category->getId());
                $sub_category = $cat->getChildrenCategories();

                foreach ($sub_category as $category) {
                    if ($category->getIsActive()
                            && isset($optionsFacetedData[$category->getId()])
                            && $this->isOptionReducesResults($optionsFacetedData[$category->getId()]['count'], $collectionSize)
                        ) 
                    {
                        $this->itemDataBuilder->addItemData(
                            $this->escaper->escapeHtml($category->getName().'second-tier'.$category->getParentId()),
                            $category->getId(),
                            $optionsFacetedData[$category->getId()]['count']
                        );
                    }
                }
        }
    }
    return $this->itemDataBuilder->build();
}
}

Basically adding a foreach loop to _getItemsData() function to display the second tier of category.