The way Magento works now, is to get the Store, and on the store, you can get the categories from the storecollection like $oStoreCollection->addCategoryFilter(array('1','2'));
I came across a solution that might help you, found here at:
http://www.magentocommerce.com/boards/&/viewthread/201114/#t329230
The code they use, looks like this:
Override Mage/Catalog/Model/Resource/Eav/Mysql4/Product/Collection, and add the following methods:
public function addCategoriesFilter($categories)
{
$this->_productLimitationFilters['category_ids'] = $categories;
if ($this->getStoreId() == Mage_Core_Model_App::ADMIN_STORE_ID) {
$this->_applyZeroStoreProductLimitations();
} else {
$this->_applyProductLimitations();
}
return $this;
}
protected function _applyProductLimitations()
{
$this->_prepareProductLimitationFilters();
$this->_productLimitationJoinWebsite();
$this->_productLimitationJoinPrice();
$filters = $this->_productLimitationFilters;
// Addition: support for filtering multiple categories.
if (!isset($filters['category_id']) && !isset($filters['category_ids']) && !isset($filters['visibility'])) {
return $this;
}
$conditions = array(
'cat_index.product_id=e.entity_id',
$this->getConnection()->quoteInto('cat_index.store_id=?', $filters['store_id'])
);
if (isset($filters['visibility']) && !isset($filters['store_table'])) {
$conditions[] = $this->getConnection()
->quoteInto('cat_index.visibility IN(?)', $filters['visibility']);
}
// Addition: support for filtering multiple categories.
if (!isset($filters['category_ids'])) {
$conditions[] = $this->getConnection()
->quoteInto('cat_index.category_id=?', $filters['category_id']);
if (isset($filters['category_is_anchor'])) {
$conditions[] = $this->getConnection()
->quoteInto('cat_index.is_parent=?', $filters['category_is_anchor']);
}
} else {
$conditions[] = $this->getConnection()->quoteInto('cat_index.category_id IN(' . implode(',', $filters['category_ids']) . ')', "");
}
$joinCond = join(' AND ', $conditions);
$fromPart = $this->getSelect()->getPart(Zend_Db_Select::FROM);
if (isset($fromPart['cat_index'])) {
$fromPart['cat_index']['joinCondition'] = $joinCond;
$this->getSelect()->setPart(Zend_Db_Select::FROM, $fromPart);
}
else {
$this->getSelect()->join(
array('cat_index' => $this->getTable('catalog/category_product_index')),
$joinCond,
array('cat_index_position' => 'position')
);
}
$this->_productLimitationJoinStore();
Mage::dispatchEvent('catalog_product_collection_apply_limitations_after', array(
'collection' => $this
));
return $this;
}
protected function _applyZeroStoreProductLimitations()
{
$filters = $this->_productLimitationFilters;
// Addition: supprot for filtering multiple categories.
$categoryCondition = null;
if (!isset($filters['category_ids'])) {
$categoryCondition = $this->getConnection()->quoteInto('cat_pro.category_id=?', $filters['category_id']);
} else {
$categoryCondition = $this->getConnection()->quoteInto('cat_pro.category_id IN(' . implode(',', $filters['category_ids']) . ')', "");
}
$conditions = array(
'cat_pro.product_id=e.entity_id',
$categoryCondition
);
$joinCond = join(' AND ', $conditions);
$fromPart = $this->getSelect()->getPart(Zend_Db_Select::FROM);
if (isset($fromPart['cat_pro'])) {
$fromPart['cat_pro']['joinCondition'] = $joinCond;
$this->getSelect()->setPart(Zend_Db_Select::FROM, $fromPart);
}
else {
$this->getSelect()->join(
array('cat_pro' => $this->getTable('catalog/category_product')),
$joinCond,
array('cat_index_position' => 'position')
);
}
return $this;
}
It then gets called like this:
$collection = Mage::getModel('catalog/product')->getCollection()
->addAttributeToSelect('*')
->distinct(true) // THIS IS WHAT YOU NEED TO ADD
->addCategoriesFilter($category->getAllChildren(true)); // Make sure you don't forget to retrieve your category here.
HTH