title: Is there a Rails way to check which attributes have been updated in an observer?
body: <p>I have an <code>ActivityObserver</code>, which is observing <code>tasks</code>, and has an <code>after_update</code> callback.</p>
  
  <p>I want to test if a particular attribute has been modified in the <code>update</code>.</p>
  
  <p>Is there a Rails way to compare the attributes of the subject with what they were before the update, or to check if they have changed?</p>