Products from two categories

Question

i am looking for product collection with specific category filter. Lets say i have products in categories:

---

Category A: Prod_1, Prod_2, Prod_3

Category B: Prod_4, Prod_5, Prod_6

Category C: Prod_7, Prod_8, Prod_9

Category D: Prod_1, Prod_5, Prod_9

and i want to get products form Category D, but with condition that this products is also in Category A , so it must return:

Prod_1 only

Answer 1

Same as @Douglas except using "in" is faster. I'd comment but I don't have enough rep.

$catAId = 1; //  category a's id
$catBId = 4; // category d's id

$productsInAandD = Mage::getModel("catalog/product")
    ->getCollection()
    ->addAttributeToSelect('*')// not optimal
    ->joinField('category_id',
                'catalog/category_product',
                'category_id',
                'product_id=entity_id',
                 null,
                'left')
    ->addAttributeToFilter("category_id" , array('in' => array($catAId , $catBId)));

Also if your only going to read attribute values and not save the product it's faster to actually specify what attributes to select.

I.e

$collection->addAttributeToSelect(array('attribute_name_1','attribute_name_3'));

Answer 2

I've renamed your categories 1,2,3,4.

$_productCollection = Mage::getModel('catalog/product')
    ->getCollection()
    ->joinField('category_id', 'catalog/category_product', 'category_id', 'product_id = entity_id', null, 'left')
    ->addAttributeToSelect('*')
    ->addAttributeToFilter('category_id', array(
        array('finset' => '1'),
        array('finset' => '4'))
    )
    ->addAttributeToSort('created_at', 'desc');