Joining to a Resource Model that doesn't have a Select?

Question

So I have this resource model that I pulled from some admin code on the dashboard. It works, but I can't seem to join to it. Doing ->getSelect()->__toString(); shows nothing? So is this not a normal collection? Why does it not work like other collections?

My guess that in order to accomplish what I want to accomplish is that I am going to have to extend the collection model, but I just wanted to see if I can get more of an idea of what is going on here.

$storeId = $this->helper('core')->getStoreId();
$collection = Mage::getResourceModel('sales/report_bestsellers_collection')
    ->setModel('catalog/product')
    ->addStoreFilter($storeId);

$collection->getSelect()->join(
        array('category_product' => 'catalog_category_product'),
        'main_table.product_id = category_product.product_id', 
        array('category_product.*')
    )->where('category_product.category_id = ?', '3')
     ->order('main_table.qty_ordered DESC')
     ->limit($limit);

Answer 1

The reporting collections work a bit differently that the Mage_Core_Model_Abstract/CRUD collections. The CRUD collections will have a SQL query assigned immediately at instantiation. The reporting collection, however, initializes its SQL statement during its load operation. You can see this in the load method down in the abstract base class for all reporting collections.

#File: app/code/core/Mage/Reports/Model/Resource/Report/Collection/Abstract.php
public function load($printQuery = false, $logQuery = false)
{
    if ($this->isLoaded()) {
        return $this;
    }
    $this->_initSelect();
    if ($this->_applyFilters) {
        $this->_applyDateRangeFilter();
        $this->_applyStoresFilter();
        $this->_applyCustomFilter();
    }
    return parent::load($printQuery, $logQuery);
}

There's no select until _initSelect is called.

The general approach you'll want to take here is

1. Create a new resource model class which extends the Mage_Reports_Model_Resource_Report_Collection_Abstract class

2. Define a _applyCustomFilter method in your class

and then use your new resource model. Your _applyCustomFilter method would look something like this.

protected function _applyCustomFilter()
{
    $return = parent::_applyCustomFilter();
    //manipulate the select all you want here (`$this->getSelect()`)
    return $return;
}

Answer 2

Looks like you're trying to load a list of "best selling" products (and filtered by a given category), I've done it in the past using the product collection from the Reports module.

Give this a shot:

$category = Mage::getModel('catalog/category')->load(3);
$report = Mage::getResourceModel('reports/product_collection')
        ->addAttributeToSelect('*')
        ->addOrderedQty()
        ->setOrder('ordered_qty', 'desc')
        ->addCategoryFilter($category);