title: Get changed attributes on after_update callback
body: <p>I'm trying to make a conditional after_update, I have the following:</p>
  
  <pre><code>  after_update do |participant|
      Rails.logger.info "#{self.previous_changes} changed."
      if self.previous_changes.include?(:current_distance)
        #Do my stuff ...
      end
    end
  </code></pre>
  
  <p>The logger prints empty hash: <code>{}</code> </p>
  
  <p>How can I check which attribute has been changed? </p>
  
  <p>I am using: <code>participant.update_attribute(:current_distance, distance)</code> to update the attribute.</p>