display product present only in both category 1 and category 2

Question

I am trying to display only those products which are present in both category 1 and category 2 or in other words only those products which are present in both parent category and its sub category I tried this:

$collection = Mage::getModel('catalog/product')
        ->getCollection()
        ->distinct(true)
        ->addAttributeToSelect('*')
        ->joinField('category_id', 'catalog/category_product', 'category_id', 'product_id = entity_id', null, 'left')
        ->addAttributeToFilter('category_id',$parentcategory)
        ->addAttributeToFilter('category_id',$subcategory ); Mage::getSingleton('catalog/product_status')->addVisibleFilterToCollection($collection);

foreach($collection as $product){ echo $product->getId(); }

EDIT:I used this to get product id but its giving error of getId on non-object

Answer 1

Well it can be achieved by intersecting the collection result arrays.

As this is not straightforward and default Magento doesn't provide any facility to intersect collections, we have to do it manually as below:

$_productCollection1 = Mage::getModel('catalog/product')
 ->getCollection()
 ->distinct(true)
 ->addAttributeToSelect('entity_id')
 ->joinField('category_id', 'catalog/category_product', 'category_id', 
    'product_id = entity_id', null, 'left')
 ->addAttributeToFilter('category_id', array('eq' => 7))
 ->addAttributeToSort('created_at', 'desc');

 foreach($_productCollection1 as $product)
    $arr1[] = $product->getId();

 $_productCollection2 = Mage::getModel('catalog/product')
 ->getCollection()
 ->distinct(true)
 ->addAttributeToSelect('entity_id')
 ->joinField('category_id', 'catalog/category_product', 'category_id', 
    'product_id = entity_id', null, 'left')
 ->addAttributeToFilter('category_id', array('eq' => 8))
 ->addAttributeToSort('created_at', 'desc');

 foreach($_productCollection2 as $product)
    $arr2[] = $product->getId();

 $result = array_intersect($arr1, $arr2);
 foreach($result as $product_id):
    $productLoaded = Mage::getModel('catalog/product')->load($product_id);
    //do something with $productLoaded
endforeach;

Now you will have only products present in both categories 7 and 8.

Hope this will help.

Answer 2

The answer should be:

$collection = Mage::getModel("catalog/product")
    ->getCollection()
    ->addAttributeToSelect('*')
    ->joinField(
        'category_id',
        'catalog/category_product',
        'category_id',
        'product_id=entity_id',
        null,
        'left'
    )
    ->addAttributeToFilter('category_id', array('in' => array(
        $id1,
        $id2
    )));
$collection->getSelect()->having('count(entity_id) > 1')->group('entity_id');