Set filter to layered navigation programmatically

Question

Using below code i'm getting list of filterable attributes

$layer = Mage::getModel("catalog/layer");
$attributes = $layer->getFilterableAttributes();

I can't able to set filters and get it's sub filters. example i have below 2 filterable attributes,

Size
 28 (1)
 30 (2)

Color
 Green (2)
 Red (1)

On selecting Size 28, my sub filter is

Color
 Red (1)

So i'm trying to achieve below,

• Get all available attribute filters initially (got this using getFilterableAttributes())

• Set filter programatically and get it's sub filters as like above example

Answer 1

To set some "initial" filters you can rewrite the app\code\core\Mage\Catalog\Model\Layer.php class and adjust the product collection in the method getProductCollection:

    $collection = parent::getProductCollection();
    $db = Mage::getSingleton('core/resource')->getConnection('core_read');
    $tableAlias = 'amasty_example';
    $conditions = array(
        "{$tableAlias}.entity_id = e.entity_id",
        $db->quoteInto("{$tableAlias}.attribute_id = ?", 198),
        $db->quoteInto("{$tableAlias}.store_id = ?", $collection->getStoreId()),
        $db->quoteInto("{$tableAlias}.value = ?", 67)
    );

    $collection->getSelect()->join(
        array($tableAlias => Mage::getSingleton('core/resource')->getTableName('catalog/product_index_eav')),
        implode(' AND ', $conditions),
        array()
    );

the code above is based on app\code\core\Mage\Catalog\Model\Resource\Layer\Filter\Attribute.php