title: Magento: how to filter the result of Mage::getResourceModel('eav/entity_attribute_collection') by Category
body: <p>I've used the follow code to get the whole list of manufacturer. </p>
  
  <pre><code>    $currentCategory = Mage::registry('current_category');
      $product = Mage::getModel('catalog/product');       
      $attributes = Mage::getResourceModel('eav/entity_attribute_collection')-&gt;setEntityTypeFilter($product-&gt;getResource()-&gt;getTypeId())-&gt;addFieldToFilter('attribute_code', 'brand_name');      
      $attribute = $attributes-&gt;getFirstItem()-&gt;setEntity($product-&gt;getResource());       
      $manufacturers = $attribute-&gt;getSource()-&gt;getAllOptions(false);      
  </code></pre>
  
  <p>Now, how can I get the manufacturer list that belong to a specific category?  I've been digging for a while to find out a way to limit the result by a category but no luck.
  Can ResourceModel filtered by Category?</p>
  
  <p>Thanks.</p>