title: filtering product collection on "is_salable"
body: <p>I would like to filter a product collection to show only items that are in stock.  I thought this would be easy given that there's an attribute called 'is_salable' that is 1 (true) if it's in stock, 0 (false) if not.  But no matter what I do, it doesn't work.  Further, it seems to halt the execution of the query before it finishes.</p>
  
  <p>Here's some sample code:</p>
  
  <pre><code>$this-&gt;_productCollection = Mage::getModel('catalog/product')-&gt;getCollection();
  $this-&gt;_productCollection-&gt;addAttributeToSelect('*');
  $this-&gt;_productCollection-&gt;addAttributeToFilter('my_attribute', true);
  //So far, so good...filtering on 'my_attribute' works!
  Mage::Log("select: " . $this-&gt;_productCollection-&gt;getSelect());
  //Successfully outputs the SQL query
  $this-&gt;_productCollection-&gt;addFieldToFilter('is_salable', '1');
  Mage::Log("select: " . $this-&gt;_productCollection-&gt;getSelect());
  //does NOT output any query...it's like it died trying
  </code></pre>
  
  <p>So what am I doing wrong?  I've tried 'addFieldToFilter', 'addAttributeToFilter', and miscellaneous other queries, such as: <code>addFieldToFilter('is_salable', array('eq' =&gt; true))</code>, etc...</p>
  
  <p>Anyone know how to do this?  If 'is_salable' is not the answer, all I need to do is filter out products that are not in stock...so whatever works to do that would be fine :)</p>
  
  <p>Thanks!</p>