title: Django: Tastypie: Updating record after displaying it
body: <p>I have a very simple model:</p>
  
  <pre><code>class challenge(models.Model):
      challenge = models.CharField(max_length=255)
      timestamp = models.DateTimeField('date published',null=True,blank=True)
      code = models.CharField(max_length=255)
  </code></pre>
  
  <p>And that model is linked via Tastypie.</p>
  
  <p>By default the timestamp field is empty when you add an entry.</p>
  
  <p>I would like to update the timestamp field with the time that the user first accessed the resource.</p>
  
  <p>For example, if admin added data is:</p>
  
  <pre><code>Challenge: Do something
  Timestamp: Null
  Code: 123
  </code></pre>
  
  <p>Then after accessing <code>/api/challenge/1/?format=json</code> to have the output be:</p>
  
  <pre><code>{"challenge": "Do something", "code": "sasdasd", "id": 2, "resource_uri": "/api/challenge/2/", "timestamp": "2015-05-11T12:18:54"}
  </code></pre>
  
  <p>Is this possible using Tastypie and how?</p>