title: confused about self.instance in save() of child of ModelForm()
body: <p>The <a href="http://docs.djangoproject.com/en/dev/topics/forms/modelforms/#the-save-method" rel="nofollow">save() documentation</a> explains that:</p>
  
  <blockquote>
    <p>A subclass of ModelForm can accept an
    existing model instance as the keyword
    argument instance; if this is
    supplied, save() will update that
    instance. If it's not supplied, save()
    will create a new instance of the
    specified model</p>
  </blockquote>
  
  <p>However, <code>self.instance</code> in <code>save()</code> always has an object.</p>
  
  <p>So, how do I tell if the <code>instance</code> is existing or a newly created one?</p>