title: Write-once fields in Django models
body: <p>I'm having a pretty hard time trying to create a write-once field in a Django model. Ideally I'd want it to work like a final variable, although I can settle for simply preventing it from being edited through the admin.</p>
  
  <p>I know there is a solution for read-only fields, but it also affects the add form, and I don't want the field to be read-only there.</p>