title: cakephp - don't change null field on update
body: <p>i use cakephp 2.5.4 and want when update record in database, fields that is null, do not change</p>
  
  <p>this is my update function</p>
  
  <pre><code>...
     if ($this-&gt;User-&gt;save($this-&gt;request-&gt;data)) {
         return $this-&gt;User-&gt;id;
     }
  ...
  </code></pre>