Magento – Get Next and Previous item from a collection

Actually getAllIds is the best way of doing it. For example in the product collection resource model the method looks like this:

public function getAllIds($limit = null, $offset = null)
{
    $idsSelect = $this->_getClearSelect();
    $idsSelect->columns('e.' . $this->getEntity()->getIdFieldName());
    $idsSelect->limit($limit, $offset);
    $idsSelect->resetJoinLeft();

    return $this->getConnection()->fetchCol($idsSelect, $this->_bindParams);
}

So everything is retrieved from a single select and no iteration is required. Also in the abstract resource model it looks like this:

public function getAllIds()
{
    $idsSelect = clone $this->getSelect();
    $idsSelect->reset(Zend_Db_Select::ORDER);
    $idsSelect->reset(Zend_Db_Select::LIMIT_COUNT);
    $idsSelect->reset(Zend_Db_Select::LIMIT_OFFSET);
    $idsSelect->reset(Zend_Db_Select::COLUMNS);

    $idsSelect->columns($this->getResource()->getIdFieldName(), 'main_table');
    return $this->getConnection()->fetchCol($idsSelect);
}

So everything that extends Mage_Core_Model_Resource_Db_Collection_Abstract should use this unless specified otherwise.

The method you looked at comes from the base class Varien_Data_Collection but it is overwritten in its children.

I don't believe there is a way to grab a specific index from Varien_Data_Collection (however I would love to be proven wrong on this!)

Notwithstanding, here are my attempts.

    /**
     * Method 1,
     * Using LIMIT to grab the 9th item, a new query for each item.
     * Inefficient but fun!
     */
    $collection = new Varien_Data_Collection();
    $collection->setPageSize(1);
    $collection->setCurPage(9);
    $ninthItem = $collection->getFirstItem();

    /**
     * Method 2,
     * the collection toArray functionality.
     *
     * It actually iterates through the collection within the toArray call,
     * but at least it saves you from doing the iteration yourself!
     */
    $collection = new Varien_Data_Collection();
    $collectionData = $collection->toArray();
    $ninthItem = $collectionData['items'][8];