How to Get Product Category IDs Without Loading the Whole Product

Question

By default, you can load product category IDs using this:

$product = Mage::getModel('catalog/product')->load($_productId);
$category_ids = $product->getCategoryIds();

But how can we load the category IDs without loading the whole product model? Note that getCategoryIds() is a function of product model not data.

Answer 1

You could do something like this:

$product = Mage::getModel('catalog/product');
$product->setId($_productId);
$categoryIds = $product->getResource()->getCategoryIds($product);

Answer 2

Just mentioning it here since this would be possible too (because getCategoryIds actually does need the product's Id (pulls it from $product->getId()) from the whole submitted model only):

$obj = new Varien_Object();
$cats = Mage::getResourceSingleton('catalog/product')->getCategoryIds($obj->setId($productId));

For the record, that's the getCategoryIds method from Mage_Catalog_Model_Resource_Product:

    /**
     * Retrieve product category identifiers
     *
     * @param Mage_Catalog_Model_Product $product
     * @return array
     */
    public function getCategoryIds($product)
    {
        $adapter = $this->_getReadAdapter();

        $select = $adapter->select()
            ->from($this->_productCategoryTable, 'category_id')
            ->where('product_id = ?', (int)$product->getId());

        return $adapter->fetchCol($select);
    }

Answer 3

relation between product and category manage it in catalog_category_product

you can use this sql query

select cc.* from catalog_category_entity cc
   join catalog_category_product cp on cc.entity_id = cp.category_id
   where cp.product_id = {{your product id}};

see Josep's answer here

Answer 4

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$product = $objectManager->get('Magento\Framework\Registry')->registry('current_product');//get current product
$categories = $product->getCategoryIds(); /*will return category ids array*/

Answer 5

Due to getCategoryIds() logic in Mage_Catalog_Model_Product it's enough to have product ID as stated before in Christoph Farnleitner answer so it can be as easy as:

Mage::getModel('catalog/product')->setId($productId)->getCategoryIds()