How to get all products from an array of specific categories and grouped by the categories

Question

I am trying to retrieve a list of product from an array of specific categories IDs and group by its category.

In this way, I retrieve all product in the root Category:

$collection = Mage::getResourceModel('catalog/product_collection')
      ->addAttributeToFilter('status', 1) // enabled
      ->addAttributeToSelect(array('name', 'price', 'small_image'))
      ->addUrlRewrite();
foreach ($collection as $item) {
    echo $item->getName();
}

Then I saw in another post, that it is possible to get products from specific category, in this way:

$storeId = Mage::app()->getStore()->getId();
$categoryIds = $catIds; Array = [23,24,25,27,30]
$productCollection = Mage::getModel('catalog/product')->getCollection();

$joinConditions = array(
  'cat_index.product_id=e.entity_id',
  $productCollection->getConnection()->quoteInto('cat_index.category_id IN (?)', $categoryIds),
  $productCollection->getConnection()->quoteInto('cat_index.store_id=?', $storeId)
);
$productCollection->getSelect()->join(
  array('cat_index' => $this->getTable('catalog/category_product_index')),
  $joinConditions,
  array('cat_index_position' => 'position')
);

$productCollection
    ->addStoreFilter()
    ->setVisibility(Mage::getSingleton('catalog/product_visibility')->getVisibleInCatalogIds());
    ->addMinimalPrice()
    ->addFinalPrice()
    ->addTaxPercents()
    ->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes())
    ->addUrlRewrite();

But unfortunately I cannot adapt this last code to my initial one. Any help would be appreciated a lot.

UPDATE

This is what I have now:

$storeId = Mage::app()->getStore()->getId();
$categoryIds = array('42','43','44','45','46','47');
$productCollection = Mage::getModel('catalog/product')->getCollection();

$joinConditions = array(
    'cat_index.product_id=e.entity_id',
    $productCollection->getConnection()->quoteInto('cat_index.category_id IN (?)', $categoryIds),
    $productCollection->getConnection()->quoteInto('cat_index.store_id=?', $storeId)
);
$productCollection->getSelect()->join(
    array('cat_index' => $this->getTable('catalog/category_product_index')),
    $joinConditions,
    array('cat_index_position' => 'position')
);

$productCollection
    ->addAttributeToFilter('status', 1)
    ->addAttributeToSelect(array('name', 'price', 'small_image'))
    ->addUrlRewrite();

foreach ($productCollection as $item) {
  echo $item->getName();
}

Unfortunatelly I have an error:

THERE HAS BEEN AN ERROR PROCESSING YOUR REQUEST Exception printing is disabled by default for security reasons.

Error log record number: 1276294659917

Answer 1

Hard to tell what you're trying to achieve. If you want to add the filtering from your previous code to the category filtered collection and remove the surplus stuff introduced by that alien code, you'll have to merge like this:

$storeId = Mage::app()->getStore()->getId();
$categoryIds = array('42','43','44','45','46','47');
$productCollection = Mage::getModel('catalog/product')->getCollection();

$joinConditions = array(
    'cat_index.product_id=e.entity_id',
    $productCollection->getConnection()->quoteInto('cat_index.category_id IN (?)', $categoryIds),
    $productCollection->getConnection()->quoteInto('cat_index.store_id=?', $storeId)
);

$productCollection->getSelect()->join(
        array('cat_index' => $productCollection->getTable('catalog/category_product_index')),
        implode(' '.Zend_Db_Select::SQL_AND.' ', $joinConditions),
        array('cat_index_position' => 'position', 'cat_id' => 'category_id')
    )
    ->order('cat_index.category_id')
;

$productCollection
    ->addAttributeToFilter('status', 1)
    ->addAttributeToSelect(array('name', 'price', 'small_image'))
    ->addUrlRewrite();

foreach ($productCollection as $item) {
    $categoryId = $item->getCatId();
    echo $item->getName();
}

If it's the grouping that needs to be done, leave a comment – and bear in mind that grouping may yield unwanted results since a product can belong to more than one category.