Product filter by category not working

Question

I have three categories like CAT-A, CAT-B, CAT-C and I want the product collection from these by the order CAT-A products, CAT-B products, CAT-C products.

When I trying to get the product collection without any filter, I get the random products from these categories.

I'm trying to filter the product collection by category like

$collection = Mage::getModel('catalog/product')->getCollection()
->addAttributeToSort('category_id', 'ASC')

but it's not working.

Answer 1

You have to join the category_product table if you want to be able to sort them because the category isn't a product attribute.
I believe there is a addCategoryFilter function that you can add to your collection, which takes a Mage_Catalog_Model_Category as an argument.
But this might prove inefficient if you have 3 categories and you might have to resort to using a join.

Answer 2

The code below might help you with JOINing categories to your product collection

$_rootcatID ='yourId';
$_testproductCollection = Mage::getResourceModel('catalog/product_collection')
  ->joinField('category_id','catalog/category_product','category_id','product_id=entity_id',null,'left')
  ->addAttributeToSort('category_id', array('in' => $_rootcatID))