title: Django generate custom ID
body: <p>I saw <a href="https://stackoverflow.com/questions/38698729/django-how-to-add-a-custom-id-column-to-identify-my-objects">this answer</a> but there is no specific answer yet. I want to create custom <code>id</code> that starts with letter. When a new record comes into database I want to change the <code>id</code> to A00001, .... A00002, .... A00010, ...A10000 etc. The <code>id</code> will be always in range <code>99999- 00001</code> so how can I do that? </p>
  
  <p><strong>my model is simple:</strong></p>
  
  <pre><code>class Custom(models.Model):
      id = models.AutoField(primary_key=True, editable=False)
  </code></pre>