title: How to add new column to order grid using observers?
body: <p>Could you help me? I'm trying to add new column to the order grid in admin using observers. 
  there is my config.xml</p>
  
  <pre><code>    &lt;adminhtml&gt;
          &lt;events&gt;
              &lt;adminhtml_block_html_before&gt;
                  &lt;observers&gt;
                      &lt;order_grid&gt;
                          &lt;type&gt;model&lt;/type&gt;
                          &lt;class&gt;Order_Grid_Model_Observer&lt;/class&gt;
                          &lt;method&gt;addItemsColumn&lt;/method&gt;
                      &lt;/order_grid&gt;
                  &lt;/observers&gt;
              &lt;/adminhtml_block_html_before&gt;
          &lt;/events&gt;
      &lt;/adminhtml&gt;
  </code></pre>
  
  <p>There is my observer code:</p>
  
  <pre><code>class Order_Grid_Model_Observer
  {
      public function addItemsColumn($observer)
      {
          $_block = $observer-&gt;getBlock();
          $_type = $_block-&gt;getType();
          if ($_type == 'adminhtml/sales_order_grid') {
              $_block-&gt;addColumn('total_item_count', array(
                  'header'=&gt; Mage::helper('sales')-&gt;__('Items'),
                  'width' =&gt; '80px',
                  'type'  =&gt; 'text',
                  'index' =&gt; 'total_item_count',
                  'sortable' =&gt; false,
                  'filter' =&gt; false
              ));
              $_block-&gt;getColumn('real_order_id')
                  -&gt;setData('filter_index', 'main_table.increment_id');
  
              $collection = $_block-&gt;getCollection();
              $collection-&gt;clear();
              $collection-&gt;getSelect()
                  -&gt;joinLeft(array('o' =&gt; 'sales_flat_order'), 'o.entity_id = main_table.entity_id', array('total_item_count'));
              $_block-&gt;setCollection($collection);
          }
      }
  }
  </code></pre>
  
  <p>And I almost did it, but when I try to sort grid by some field, I get the error "Column 'increment_id' in field list is ambiguous". It's weird because I've updated 'filter_index' for 'increment_id' field. Any ideas why collection for this block was not updated? Thanks.</p>