Detailed description of getting values for product collection

Question

I am interested in the exact process of getting the values for a product collection.

If I have a product collection

$products = Mage::getModel('catalog/product')->getCollection();

it will just provide basic product attributes like sku.

Example from a $product->debug()

Array
(
    [entity_id] => 3221
    [entity_type_id] => 4
    [attribute_set_id] => 9
    [type_id] => simple
    [sku] => 12345678
    [has_options] => 1
    [required_options] => 1
    [created_at] => 2015-01-01 15:20:27
    [updated_at] => 2015-02-27 11:24:49
    [cat_index_position] => 20009
    [is_salable] => 1
    [stock_item (Varien_Object)] => Array
        (
            [is_in_stock] => 1
        )

)

When I need more values I can use addAttributeToSelect() to extend the information I get from my collection. E.g. I mostly use

$products->addAttributeToSelect(Mage::getSingleton('catalog/config')->getProductAttributes())

because it is not the "I don't care, just gimme everything" hammer like '*' and mostly fits my needs and it works very well. The $product->debug() contains much more values after adding this.

What I don't understand is the mechanism behind it. When I use $products->load(false,true) to dump the sql, there is no hint to the extra attributes.

SELECT `e`.*, `cat_index`.`position` AS `cat_index_position` 
FROM `catalog_product_entity` AS `e`
INNER JOIN `catalog_category_product_index` AS `cat_index` 
  ON cat_index.product_id=e.entity_id 
    AND cat_index.store_id=1 
    AND cat_index.visibility IN(2, 4) 
    AND cat_index.category_id = '2' 
ORDER BY `e`.`entity_id` ASC

I went down to \Mage_Eav_Model_Entity_Collection_Abstract::addAttributeToSelect but I am not sure where to go from there.

Somewhere between getCollection() and a foreach($products as $product) the values are added and I really would like to know where and how the magic happens :)

Can somebody explain this to me?

And to extend my question: Where are the borders of the addAttributeToSelect() function? It seems it can do some joining but where is the point you need to do it yourself with joinAttribute()? Hope I could make myself clear with this.

Thanks in advance! :)

Answer 1

Using the collection in a foreach loop causes the collection to be loaded (see Varien_Data_Collection for which functions trigger load). The load function of Varien_Data_Collection is overloaded by Mage_Eav_Model_Entity_Collection_Abstract, which is part of the inheritance chain of the product collection (Mage_Catalog_Model_Resource_Product_Collection). Somewhere in the middle of the load method it triggers Mage_Eav_Model_Entity_Collection_Abstract::loadAttributes which is responsible for hydrating all the attribute data.