title: variables as parameters in field options
body: <p>I want to create a model, that will set editable=False on creation, and editable=True on editing item. I thought it should be something like this: </p>
  
  <pre><code> home = models.ForeignKey(Team, editable=lambda self: True if self.id else False)
  </code></pre>
  
  <p>But it doesn't work. Maybe something with overriding the <strong>init</strong> can help me, but i don't sure what can do the trick. I know i can check for self.id in save() method, but is too late, i want this kind of logic in admin app when im filling the fields.</p>