title: Update one attribute of a Symfony doctrine object when another changes
body: <p>In a symfony 1.4 project I have Doctrine object <code>book</code> that has a number of attributes including <code>owner</code> and <code>owner_updated_at</code>. I would like to have the field <code>owner_updated_at</code> set to the current date and time any time the value of <code>owner</code> is changed. How can I do this? </p>