title: Django model foreign key removal
body: <p>I have following model:</p>
  
  <pre><code>class Client(models.Model):
      user = models.OneToOneField(DjangoUser, unique=True)
      address = models.ForeignKey(Address,blank=True)
  
  class Address(models.Model):
      (...)
  </code></pre>
  
  <p>Then I do:</p>
  
  <pre><code>client=Client()
  client.address=address #any Address instance
  client.save()
  </code></pre>
  
  <p>And now: how can I remove foreign association key from client?</p>
  
  <pre><code>client.address=None 
  </code></pre>
  
  <p>seem not to work.</p>