Programmatically Change Total Weight in Cart

Question

I need to override the total weight of a customers order programmatically based on various conditions.

I am wanting to change the weight of the whole cart from the real value of 5000 to 957565:

// first echo (to check if the weight is actually saved)
echo $quote->getShippingAddress()->getWeight().'<br />';

if ($x == 'y' && $a < $b)
{
    $quote = Mage::getSingleton('checkout/session')->getQuote();
    $quote->getShippingAddress()->setWeight(957565);
    $quote->save();

    // second echo
    echo $quote->getShippingAddress()->getWeight();
}

Which outputs each time I visit the cart page/another page:

5000
957565

So the weight does seem to be saved correctly when outputting the second echo but when I reload the page or go to the checkout the weight always reverts back to 5000

Answer 1

The Quote gets recalculated at various stages during a customer's journey through to a completed order. The weight would get recalculated from the individual items in the quote which is why your changes don't survive a reload.

A possibly better approach would be to adjust the weight of an individual quote item or make sure that your code gets executed at the right time, using the event sales_quote_collect_totals_after for example.