title: Magento addAttributeToFilter
body: <p>I'm using attributes to a product, <code>availablepostalcodes</code>. This is a multi-select, there are 3 options per product:
  1234
  2345
  3456</p>
  
  <p>Now after a post (form) on the categorypage i've <code>$_POST['postalcode']</code>, this contains 2345. But when i do this nothing will filter:</p>
  
  <pre><code>$_productCollection=$this-&gt;getLoadedProductCollection();
  $_productCollection-&gt;addAttributeToFilter("availablepostalcodes", array("finset"=&gt;"5021"));
  
  
  echo $_productCollection-&gt;load()-&gt;count(); 
  </code></pre>
  
  <p>The echo still shows 5, however it should be 1, cause one product has that available postalcode. </p>
  
  <p>This is the product-list view <code>/catalog/product/list.phtml</code></p>
  
  <p>What do i miss or don't see?</p>
  
  <p><strong>Edit:</strong></p>
  
  <pre><code>$_productCollection = Mage::getModel('catalog/product')
                                -&gt;getCollection()
                                -&gt;addAttributeToSelect('availablepostalcodes')
                                -&gt;addAttributeToFilter('entity_id', array('in' =&gt; $_productCollection-&gt;getAllIds()));  
  </code></pre>
  
  <p>Then i get all the products again.</p>
  
  <p>But when i do <code>$_productCollection-&gt;addAttributeToFilter("availablepostalcodes", array("in" =&gt; array(5021')));</code> it still doesn't work</p>