title: Get earliest object by field in django
body: <p>I have <code>date_updated</code> field in my model. I want to get object whose <code>date_updated</code> time is oldest or is null/blank 
  How can I do this most efficient way?</p>
  
  <pre><code>class Product(models.Model):
      date_updated = models.DateTimeField(auto_now_add=False, auto_now=True, null=True)
      date_added = models.DateTimeField(auto_now_add=True, auto_now=False)
  </code></pre>