title: change model property by another property django
body: <p>I have model <code>Word</code></p>
  
  <pre><code>class Word(models.Model): 
     group_id = models.IntegerField()
     old_group_id = models.IntegerField(default=0)
  </code></pre>
  
  <p>I have some word query:</p>
  
  <pre><code>words = Word.objects.filter(...)
  </code></pre>
  
  <p>Right now I want every word to set  <code>old_group_id</code> to <code>group_id</code> 
  something like <code>words.update(old_group_id=group_id)</code>
  So far I am doing it by iterating through every word, but this doesn't feel very effective:</p>
  
  <pre><code>for word in words:
    word.old_group_id = word.group_id
    word.save()
  </code></pre>