Magento 2 How to load product collection, Category collection, Load product by child ID

Question

I have some experience in M1 and M2 but am somewhat struggling to grasp how to load the child category. At the minute it loads the default M2 category with [0] or [1] next category etc. But I just want to load the last category the product is attached too. Imagine Default > Electrics > Stereo > Cables. I just want to load the cables version, I know I could do it by doing [3] but imagine if another category only had Default > Electrics > Stereo it would then be [2] and I need that logic to be dynamic.

Pease can someone help, I've been crawling for hours and getting more and more confused.

This code relates to a custom related products module which is built to simply load products in the same last category view.

class Related extends Template { protected $_productCollectionFactory; protected $_categoryFactory; protected $_registry;

public function __construct(
    \Magento\Catalog\Model\ResourceModel\Product\CollectionFactory $productCollectionFactory,
    \Magento\Catalog\Model\CategoryFactory $categoryFactory,
    \Magento\Framework\View\Element\Template\Context $context,
    \Magento\Framework\Registry $registry,
    \Magento\Catalog\Model\CategoryRepository $categoryRepository
)
{
    $this->_categoryFactory = $categoryFactory;
    $this->_productCollectionFactory = $productCollectionFactory;
    $this->_registry = $registry;
    $this->categoryRepository = $categoryRepository;
    parent::__construct($context);
}


public function getRelatedProducts()
{


    $categoryIds = $this->_registry->registry('current_product')->getCategoryIds();


    $category = $this->_categoryFactory->create()->load($categoryIds[0]);
    $collection = $this->_productCollectionFactory->create();
    $collection->addAttributeToSelect('*')
        ->addCategoryFilter($category)
        ->addAttributeToFilter('visibility', \Magento\Catalog\Model\Product\Visibility::VISIBILITY_BOTH)
        ->addAttributeToFilter('status', \Magento\Catalog\Model\Product\Attribute\Source\Status::STATUS_ENABLED)
        ->setPageSize(5);
    return $collection;


}

}

Answer 1

Why dont you just get count and there you will find your last item id ?

some thing like this :

$categoryIds = $this->_registry->registry('current_product')->getCategoryIds();    
$catId = $categoryIds[count($categoryIds) - 1]?

It should work

Answer 2

To find dynamically the last item of the array, the end php function should help you. For instance,

$categoryIds = [1, 3, 5];
$catId = end($categoryIds);

this would return $catId = 5 and this would adjust itself if the categoryIds had more values or less.