title: How add custom filter to colum grid in Magento?
body: <p>i have custom grid in Adminhtml.</p>
  
  <pre class="lang-php prettyprint-override"><code>protected function _prepareCollection()
  {
      /* @var $collection Mage_Catalog_Model_Resource_Product_Collection */
      $collection = Mage::getModel('catalog/product')-&gt;getCollection();
      $collection-&gt;addAttributeToSelect('*');
      $collection-&gt;joinField('category_id', 'catalog/category_product', 'category_id', 'product_id=entity_id', null, 'left');
      $collection-&gt;groupByAttribute('entity_id');
      $collection-&gt;addStaticField('category_id');
      $collection-&gt;addExpressionAttributeToSelect('category_grp', 'GROUP_CONCAT(category_id)', 'category_id');
  
      $this-&gt;setCollection($collection);
      return parent::_prepareCollection();
  }
  
  protected function _prepareColumns()
  {
      parent::_prepareColumns();
  
   $this-&gt;addColumn('category_id', array(
          'header'    =&gt; Mage::helper('newsletter')-&gt;__('Category'),
          'index'     =&gt; 'category_grp',
          'type'      =&gt; 'categories',
          'options'   =&gt; $options,
          'align'     =&gt; 'left',
  //      'filter_index' =&gt; $this-&gt;_getFlatExpressionColumn('category'),
      return $this;
  }
  </code></pre>
  
  <p>category_grp is array with int</p>
  
  <p>My question is how can i add filter to header column which filters the item from the field?</p>
  
  <p>for example filter only product with category_id=7 (category_grp is 3,6,7,13)... </p>