title: Override a form in Django admin
body: <p>In Django admin I want to override and implement my own form for a model (e.g. Invoice model).</p>
  
  <p>I want the invoice form to have auto-fill fields for customer name, product name and I also want to do custom validation (such as credit limit for a customer). How can I override the default form provided by Django admin and implement my own?</p>
  
  <p>I am new to Django, I appreciate any pointers.  </p>