custom_autofield.py

class Author(models.Model):
  name = models.CharField(max_length=200)

class Book(models.Model):
  title = models.CharField(max_length=200)
  author = models.ForeignKey(Author)
  # here it is:
  # this should start from 1 for the same author
  # examples:
  # author1 | 1
  # author1 | 2
  # author2 | 1
  # author1 | 3
  # author2 | 2
  # ...
  sort_id = models.AutoField()