title: Django: Populate user ID when saving a model
body: <p>I have a model with a created_by field that is linked to the standard Django User model.  I need to automatically populate this with the ID of the current User when the model is saved.  I can't do this at the Admin layer, as most parts of the site will not use the built-in Admin.  Can anyone advise on how I should go about this?</p>