title: Validators before_update(ing) a model
body: <p>I want when editing a model to perform some validations so I thought that the best way is to use the </p>
  
  <pre><code>before_update
  </code></pre>
  
  <p>in Rails.</p>
  
  <p>Can someone provide an example of a custom validator using a before_update call?</p>
  
  <p>For example:</p>
  
  <p>I have a class Topic and I want to allow only the creator of the Topic to be able to change the title.</p>