title: How to do a ORDER BY with an expression in Magento Collections
body: <p>I already know how to do a simple order by with a Magento collection. But this time I want to do something like this,</p>
  
  <pre><code>SELECT * FROM table WHERE filed LIKE '%abc%' 
  ORDER BY  SUBSTRING(field, 1, 2) DESC LIMIT 10
  </code></pre>
  
  <p>So how can I add SUBSTRING function to my order by clause? Any ideas? </p>