title: Django, updating from model
body: <p>assume I have this little model:</p>
  
  <pre><code>class Deal(models.Model):
      purchases = models.IntegerField(default=0)#amount of purchases so far
  
      increase_purchases(self,to_add):
          self.update( purchases =self.purchases + to_add)
  </code></pre>
  
  <p>when I try to use this increase_purchases model from shell:</p>
  
  <pre><code>&gt;&gt;&gt; x = Deal.objects.get(id=1)
  &gt;&gt;&gt; x.increase_purchases(4)
  AttributeError: 'Deal' object has no attribute 'update'
  </code></pre>
  
  <p>How can I write a proper function to the model so that I can update the selected querys purchases as I want ?</p>