title: Custom model field change base of id
body: <p>I am trying to get a base 36 string to use in my URLs.  I have written a function that converts a number to base 36, using letters to represent digits higher than 9.
  I am trying to find a good way to set the default value of a character field equal to this function, but I am not sure how to get the Django default primary key value into it.  Any ideas?</p>
  
  <p>I would think I could just put something like this into my model, but it does not work:</p>
  
  <p><code>base_36 = models.TextField(default=convert_base(id, 36))</code></p>