title: is_unique for codeigniter form validation
body: <p>I'm trying to figure out how I can use the <code>is_unique</code> rule from the Codeigniter form validation library in the following situation.</p>
  
  <p>I'm trying to submit a edit user form and have the rule:</p>
  
  <pre><code>$this-&gt;form_validation-&gt;set_rules('user_name', 'User Name', 'required|trim|xss_clean|is_unique[users.user_name]');
  </code></pre>
  
  <p>What if other values in the form are being changed but this value stays the same. The form is going to see that this value already exists so how would I protect it from editing if this value isn't changed.</p>