How to use addFieldToFilter() in field generated in sql?

Question

If I try to filter a field in the existing table works, but a field generated by max () does not work, even renaming, display the error getBackend().

Examples:

observer

$collection->getSelect()->columns(array('last_order' => 
            new Zend_Db_Expr ("(SELECT MAX(created_at) FROM " . 
            $collection->getTable('sales/order') . " WHERE customer_id = e.entity_id)")));

grid - _prepareCollection()

$this->getCollection()->addFieldToFilter('last_order', 1);

I was wondering how can I make this filter work with my select

Answer 1

Here is an answer I gave ages ago on stackoverflow which deals with exactly that issue.

https://stackoverflow.com/a/9531624/485589

The gist of it is that when you add a column with a computed value, you also need to supply the SQL filter syntax for it.
There are several ways that can be done, all outlined in the post linked to above.