title: Filter count column Magento Customer Admin grid with filter_condition_callback
body: <p>I've modified the magento customer admin grid in <code>../app/code/local/Mage/Adminhtml/Block/Customer/Grid.php</code> to include total orders, but need to apply a filter, I've been working with filter_condition_callback but can't get it work. Any suggestions?</p>
  
  <p><strong>Modified Collection:</strong></p>
  
  <pre><code>    $collection-&gt;getSelect()-&gt;joinLeft(array('sales_flat_order'), 'sales_flat_order.customer_id=e.entity_id', array('order_total' =&gt; 'COUNT(customer_id)'));
      $collection-&gt;groupByAttribute('entity_id')
  </code></pre>
  
  <p><strong>Order Total Column:</strong></p>
  
  <pre><code>        $this-&gt;addColumn('order_total', array(
          'header' =&gt; Mage::helper('customer')-&gt;__('Total Orders'),
          'type' =&gt; 'text',
          'index' =&gt; 'order_total',
          //'filter' =&gt; true,
          'sortable' =&gt; false,
          'filter_condition_callback' =&gt; array($this, '_orderTotalFilter'),
      ));
  </code></pre>
  
  <p><strong>OrderTotalFilter Callback:</strong></p>
  
  <pre><code>    protected function _orderTotalFilter($collection, $column) {
      if (!$value = $column-&gt;getFilter()-&gt;getValue()) {
          return $this;
      }
  
          $this-&gt;getCollection()-&gt;getSelect()-&gt;where('condition = ?', "$value");
  //        $query = $collection-&gt;getSelect()-&gt;__toString();
  //        echo $query;
  
          return $this;
      }
  </code></pre>