How to cache a collection in Magento?

I have a collection that takes significant time to load. What I would like is to cache it (APC, Memcache). It is not possible to cache the entire object (as it cannot be unserialized and it is over 1 MB). I'm thinking that caching the collection data ($col->getData() ) is the way to go, but I found no way to rebuild the object based on this array. Any clues?

Collections already have some caching built in but they need a little prompting so put this in the constructor of a collection:

$cache = Mage::app()->getCacheInstance();
$prefix = "SomeUniqueValue";
$this->initCache($cache, $prefix, array(Mage_Catalog_Model_Product::CACHE_TAG));

Choose tags appropriate to the content of the collection so that it will be flushed automatically. This way builds an ID based on the query being executed, it is most useful when the collection is filtered, ordered or paged - it avoids a version conflict.

Generally this hardly gets used because when you retrieve data you almost always end up displaying it, probably as HTML, so it makes sense to cache the output instead. Block caching is widely used and better documented.

I really don't know, but I searched for all the files that have the word "cache" in them with file names of "Collection.php" and got a few results. The most promising example to look at might be Mage_Sales_Model_Entity_Quote_Item_Collection (_getProductCollection() method). Looks like Varien_Data_Collection (which is a parent class of any magento collection) has a few cache-related methods: initCache() and _getCacheInstance().

Can't say I have used them before but might be useful someday.

Good luck.

You can get more information here: Can I use Magento's Caching layer as a Key/Value Store?

I'll be posting more info there as I find it.