title: How to skip column when persisting in Doctrine2?
body: <p>I have one column for the time when object is <code>created</code> and one for when it is <code>updated</code>.
  When I create and persist new object I get the error from MySQL:</p>
  
  <blockquote>
    <p>updated cannot be null.</p>
  </blockquote>
  
  <p>I didn't set any value to it because I want <code>updated</code> column to remain untouched and eventually get default database value whatever it is.</p>
  
  <p>How to do tell doctrine to persist only those columns which values I explicitly set/changed?</p>