Magento sort by special price (disounted product) on product list page dropdown (toolbar.phtml)

Question

I want to extend sort by drop down for special price or discounted product from toolbar.phtml

<div class="sort-by" style="float:left">
    <fieldset class="sort-by">
      <label><?php echo $this->__('Sort by') ?></label>
        <select onchange="setLocation(this.value)">
          <option value="<?php echo $this->getOrderUrl('price', 'asc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Price - Low to High </option>
          <option value="<?php echo $this->getOrderUrl('price', 'desc') ?>"<?php if($this->isOrderCurrent('price') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Price - High to Low </option>
          <option value="<?php echo $this->getOrderUrl('name', 'asc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'asc'): ?> selected="selected"<?php endif; ?>> Name - A to Z </option>
          <option value="<?php echo $this->getOrderUrl('name', 'desc') ?>"<?php if($this->isOrderCurrent('name') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>> Name - Z to A </option>
         <option value="<?php echo $this->getOrderUrl('update_at', 'desc') ?>"<?php if($this->isOrderCurrent('update_at') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>>Newest First</option>
        </select>
  </fieldset>
</div>

How I can add one more option for discount so products will sort by special price ASC?

Answer 1

You should only need to insert the sort order, and the collection should already have the special price - add this to your <select>:

<option value="<?php echo $this->getOrderUrl('special_price', 'asc') ?>"<?php if($this->isOrderCurrent('special_price') && $this->getCurrentDirection() == 'desc'): ?> selected="selected"<?php endif; ?>>Special Price</option>

The only flaw in this method is that it won't filter items from the collection that are not currently on sale, though it will sort them.