How to view related products for all products in compare list?

Question

I want to add the related products block to the compare list, however I am getting a fatal error Call to a member function getRelatedProductCollection() on a non-object.

I suspect this is because I am trying to view related products on a non-product page, therefore Mage::registry('product') is null.

How do I add related products for all products in my compare list in one block?

[edit]

I have decided to extend the related products block and create the collection myself.

Doing a var_dump on $this->_itemCollection->count() is giving me 0.

protected function _prepareData() {
    //compare products list
    $_products = $this->getLayout()->getBlockSingleton('Mage_Catalog_Block_Product_Compare_List')->getItems();

    //get related products for all products in compare list
    //more or less copied from @see Mage_Catalog_Block_Product_List_Related::_prepareData()
    $this->_itemCollection = Mage::getModel('catalog/product')->getLinkInstance()
        ->useRelatedLinks()
        ->setGroupBy()
        ->getProductCollection()
        ->setIsStrongMode()
        ->addAttributeToSelect('required_options')
        ->setPositionOrder()
        ->addStoreFilter()
        ->addProductFilter($_products)
    ;
    var_dump($this->_itemCollection->count());die; //returns 0
    $this->_addProductAttributesAndPrices($this->_itemCollection);
    Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($this->_itemCollection);

    $this->_itemCollection->load();

    foreach ($this->_itemCollection as $product) {
        $product->setDoNotUseCategoryId(true);
    }

    return $this;
}

By trial and error, when I remove setIsStrongMode and addProductFilter the collection count returns my total product count.

For testing purposes, only one of my products has related products so I know the expected output should be '3' (this product has three related products).

Answer 1

In order to get this working, I am doing a foreach over each product in the compare list, fetching their related products, and adding each related product to the final collection:

protected function _prepareData() {
    $_products = $this->getLayout()->getBlockSingleton('Mage_Catalog_Block_Product_Compare_List')->getItems();

    foreach($_products as $_product) {
        $collection = $_product->getRelatedProductCollection()
            ->addAttributeToSelect('required_options')
            ->setPositionOrder()
            ->addStoreFilter();

        if(is_null($this->_itemCollection)) {
            $this->_itemCollection = $collection;
            continue;
        }

        foreach($collection as $item) {
            if(!$this->_itemCollection->getItemById($item->getId())) {
                $this->_itemCollection->addItem($item);
            }
        }
    }

    $this->_addProductAttributesAndPrices($this->_itemCollection);
    Mage::getSingleton('catalog/product_visibility')->addVisibleInCatalogFilterToCollection($this->_itemCollection);

    $this->_itemCollection->load();

    foreach ($this->_itemCollection as $product) {
        $product->setDoNotUseCategoryId(true);
    }

    return $this;
}

Not ideal as each loop performs another database call...

Still wanting a solution based on the OP.