Magento2 – How to Get Product Collection with Manufacturer Attribute Filter

If it helps anyone, this is the only way I've managed to get all products, including out of stock products, in a loop for updating the stock values.

I needed to use ->getAllIds() to get an array of all of the products IDs, and then load each product individually rather than loop through the whole collection.

$productCollection = $objectManager->create('Magento\Catalog\Model\ResourceModel\Product\CollectionFactory');
$productCollectionIds = $productCollection->create()->getAllIds();

foreach ($productCollectionIds as $productId) {
    try {
        $product = $objectManager->create('Magento\Catalog\Model\Product')->load($productId);
        // do actions with product
    } catch (Exception $e) {
        $e->getMessage();
    }
}

Did this using plain magento sql query.

class Index extends \Magento\Framework\View\Element\Template
{


protected $_resource;

public function __construct(
    Context $context,
    array $data,
    \Magento\Framework\App\ResourceConnection $resource
) {
    $this->_resource                 = $resource;
    parent::__construct($context, $data);
}

public function getResult()
{
    $connection = $this->_resource->getConnection(\Magento\Framework\App\ResourceConnection::DEFAULT_CONNECTION);

    $result = $connection->fetchAll("SELECT DISTINCT cat.value FROM catalog_product_entity_varchar as cat 
            JOIN eav_attribute as eav ON eav.attribute_id=cat.attribute_id
            WHERE eav.attribute_code='manufacturer'");
}