How to display products from subcategory to parent category in opencart?

the vqmod method is the best and works easily. just add that file and anything.xml and place it in vqmod > xml

catalog/controller/product/category.php

For Opencart Version 2.1.0.2, the solution of Ignacio works fines as well just that:

$data (of version 1.5.x) is now called

$filter_data (line #169)

Then just add the line

'filter_sub_category' => true,

after line #170 ('filter_category_id' => $category_id,)

Thanks Ignacio!

Another solution, rather than modifying the core files directly, is to use vQmod to modify the file for you. That way when you upgrade to a new version, you won't have to re-install any custom modifications you've made.

Below is an the code you'd use to accomplish this in vQmod:

<?xml version="1.0" encoding="UTF-8"?>
<modification>
    <id>Display products in sub-categories while browsing a parent category</id>
    <version>1.0.0</version>
    <vqmver>2.4.0</vqmver>
    <author>Jay Williams - [email protected]</author>
    <file name="catalog/controller/product/category.php">
        <operation>
            <search position="after"><![CDATA['filter_category_id' => $category_id,]]></search>
            <add><![CDATA['filter_sub_category' => true,]]></add>
        </operation>
    </file>
</modification>

Source: https://gist.github.com/jaywilliams/8044763

After reading the source, I figured it out:

In catalog/controller/product/category.php (or wherever you're calling function model_catalog_product->getProducts) you have to add filter_sub_category = true:

$data = array(
    'filter_category_id' => $top_category,
    'filter_sub_category' => true,
    'sort'               => $sort,
    'order'              => $order,
    'start'              => ($page - 1) * $limit,
    'limit'              => $limit
);

$product_total = $this->model_catalog_product->getTotalProducts($data);

Make sure you check the other answers if you're using a later version ;)