title: Magento observer (sales_order_grid_collection_load_before), filter collection by a product attribute
body: <p>So I'm working with <code>sales_order_grid_collection_load_before</code> observer event at the moment, where I can get the collection being used through <code>$collection = $observer-&gt;getEvent()-&gt;getOrderGridCollection();</code>, I'm just wondering, if it is possible to filter this collection by a product from the order attribute.
  What I mean with that is the order grid collection has sub products related to that order, I need to only show orders if at least one of the products match a specific criteria (in my case, I've given the products an admin_id attribute, which is set to the administrator who added the product).</p>
  
  <p>Thanks!</p>