Filter magento product collection by category id but NOT category children

Question

I am retrieving a product collection and filtering the list by category.

I have just had to add some sub categories to one of them and now the products in the sub-categories are appearing in the collection for the parent category and I cannot work out how to only show products directly assigned to the parent category.

Here is an example of the code:

$cat = Mage::getModel("[custom-namepace]/Categories"); //this just extends the native Categories model
$cat->load($catId);  //some number
$products = Mage::getResourceModel("catalog/product_collection")
                ->setStoreId(Mage::app()->getStore()->getId())
                ->addUrlRewrite()  
                ->addCategoryFilter($cat)
                ->setOrder("name","asc");

I am unable to add a filter on category_id as that causes an error. I have tried:

->addAttributeToFilter("category_id",$cat->getId())

and

->addAttributeToFilter("category_id",array("eq"=>$cat->getId()))

and

->addAttributeToFilter("category_id",array("in"=>$cat->getId()))

These all throw a fatal error like.

Fatal error: Call to a member function getBackend() on a non-object in [...]\app\code\core\Mage\Eav\Model\Entity\Abstract.php on line 816

Any pointers on how to avoid picking up products in child categories?

UPDATE

I have now also tried to get the products list through the category and I still get products from the child categories.

$products = $cat->getProductCollection()
                ->addAttributeToSelect("name")
                ->addUrlRewrite()
                ->setOrder("name","asc");

Answer 1

you could do something like this:

$collection = Mage::getResourceModel('catalog/product_collection')
->joinField('category_id',
    'catalog/category_product',
    'category_id',
    'product_id = entity_id',
    null)
->addFieldToFilter('category_id', 3);