cmd

$results = $this->model_catalog_product->getProducts($data);

$sql = "SELECT p.product_id, (SELECT AVG(rating) AS total FROM " . DB_PREFIX . "review r1 WHERE r1.product_id = p.product_id AND r1.status = '1' GROUP BY r1.product_id) AS rating, (SELECT COUNT(*) FROM " . DB_PREFIX . "product_related pr WHERE pr.product_id = p.product_id) AS related, (SELECT price FROM " . DB_PREFIX . "product_discount pd2 WHERE pd2.product_id = p.product_id AND pd2.customer_group_id = '" . (int)$customer_group_id . "' AND pd2.quantity = '1' AND ((pd2.date_start = '0000-00-00' OR pd2.date_start < NOW()) AND (pd2.date_end = '0000-00-00' OR pd2.date_end > NOW())) ORDER BY pd2.priority ASC, pd2.price ASC LIMIT 1) AS discount, (SELECT price FROM " . DB_PREFIX . "product_special ps WHERE ps.product_id = p.product_id AND ps.customer_group_id = '" . (int)$customer_group_id . "' AND ((ps.date_start = '0000-00-00' OR ps.date_start < NOW()) AND (ps.date_end = '0000-00-00' OR ps.date_end > NOW())) ORDER BY ps.priority ASC, ps.price ASC LIMIT 1) AS special"; 

foreach ($query->rows as $result) {
			$product_data[$result['product_id']] = $this->getProduct($result['product_id']);
		}

О, чудесный OpenCart! О, божественная архитектура!

Значит так... Забыли про getProducts. Вместо того, чтобы редактировать метод getProducts нужно редактировать getProduct по аналогии т.е. 

$query = $this->db->query("SELECT DISTINCT *, pd.name AS name, p.image, m.name AS manufacturer, (SELECT price FROM " . DB_PREFIX . "product_discount pd2 WHERE pd2.product_id = p.product_id AND pd2.customer_group_id = '" . (int)$customer_group_id . "' AND pd2.quantity = '1' AND ((pd2.date_start = '0000-00-00' OR pd2.date_start < NOW()) AND (pd2.date_end = '0000-00-00' OR pd2.date_end > NOW())) ORDER BY pd2.priority ASC, pd2.price ASC LIMIT 1) AS discount, (SELECT COUNT(*) FROM " . DB_PREFIX . "product_related pr WHERE pr.product_id = p.product_id) AS related, (SELECT price FROM " . DB_PREFIX . "product_special ps WHERE ps.product_id = p.product_id AND ps.customer_group_id = '" . (int)$customer_group_id . "' AND ((ps.date_start = '0000-00-00' OR ps.date_start < NOW()) AND (ps.date_end = '0000-00-00' OR ps.date_end > NOW())) ORDER BY ps.priority ASC, ps.price ASC LIMIT 1) AS special, (SELECT points FROM " . DB_PREFIX . "product_reward pr WHERE pr.product_id = p.product_id AND customer_group_id = '" . (int)$customer_group_id . "') AS reward, (SELECT ss.name FROM " . DB_PREFIX . "stock_status ss WHERE ss.stock_status_id = p.stock_status_id AND ss.language_id = '" . (int)$this->config->get('config_language_id') . "') AS stock_status, (SELECT wcd.unit FROM " . DB_PREFIX . "weight_class_description wcd WHERE p.weight_class_id = wcd.weight_class_id AND wcd.language_id = '" . (int)$this->config->get('config_language_id') . "') AS weight_class, (SELECT lcd.unit FROM " . DB_PREFIX . "length_class_description lcd WHERE p.length_class_id = lcd.length_class_id AND lcd.language_id = '" . (int)$this->config->get('config_language_id') . "') AS length_class, (SELECT AVG(rating) AS total FROM " . DB_PREFIX . "review r1 WHERE r1.product_id = p.product_id AND r1.status = '1' GROUP BY r1.product_id) AS rating, (SELECT COUNT(*) AS total FROM " . DB_PREFIX . "review r2 WHERE r2.product_id = p.product_id AND r2.status = '1' GROUP BY r2.product_id) AS reviews, p.sort_order FROM " . DB_PREFIX . "product p LEFT JOIN " . DB_PREFIX . "product_description pd ON (p.product_id = pd.product_id) LEFT JOIN " . DB_PREFIX . "product_to_store p2s ON (p.product_id = p2s.product_id) LEFT JOIN " . DB_PREFIX . "manufacturer m ON (p.manufacturer_id = m.manufacturer_id) WHERE p.product_id = '" . (int)$product_id . "' AND pd.language_id = '" . (int)$this->config->get('config_language_id') . "' AND p.status = '1' AND p.date_available <= NOW() AND p2s.store_id = '" . (int)$this->config->get('config_store_id') . "'");

UPDATE 2:

В этом прекрасном полотне надо добавить 

if ($query->num_rows) {
			return array(
				'product_id'       => $query->row['product_id'],
                                'related' => $query->row['related'], // вот это добавлено
				'name'             => $query->row['name'],
				'description'      => $query->row['description'],
				'meta_description' => $query->row['meta_description'],
				'meta_keyword'     => $query->row['meta_keyword'],
				'tag'              => $query->row['tag'],
				'model'            => $query->row['model'],
				'sku'              => $query->row['sku'],
				'upc'              => $query->row['upc'],
				'ean'              => $query->row['ean'],
				'jan'              => $query->row['jan'],
				'isbn'             => $query->row['isbn'],
				'mpn'              => $query->row['mpn'],
				'location'         => $query->row['location'],
				'quantity'         => $query->row['quantity'],
				'stock_status'     => $query->row['stock_status'],
				'image'            => $query->row['image'],
				'manufacturer_id'  => $query->row['manufacturer_id'],
				'manufacturer'     => $query->row['manufacturer'],
				'price'            => ($query->row['discount'] ? $query->row['discount'] : $query->row['price']),
				'special'          => $query->row['special'],
				'reward'           => $query->row['reward'],
				'points'           => $query->row['points'],
				'tax_class_id'     => $query->row['tax_class_id'],
				'date_available'   => $query->row['date_available'],
				'weight'           => $query->row['weight'],
				'weight_class_id'  => $query->row['weight_class_id'],
				'length'           => $query->row['length'],
				'width'            => $query->row['width'],
				'height'           => $query->row['height'],
				'length_class_id'  => $query->row['length_class_id'],
				'subtract'         => $query->row['subtract'],
				'rating'           => round($query->row['rating']),
				'reviews'          => $query->row['reviews'] ? $query->row['reviews'] : 0,
				'minimum'          => $query->row['minimum'],
				'sort_order'       => $query->row['sort_order'],
				'status'           => $query->row['status'],
				'date_added'       => $query->row['date_added'],
				'date_modified'    => $query->row['date_modified'],
				'viewed'           => $query->row['viewed']
			);