title: Add number field to Django model
body: <p>I have a django movie model</p>
  
  <pre><code>class Film(models.Model):
  
      title = models.CharField(max_length=200)
      movie_id = models.CharField(max_length=8, unique=True, primary_key=True)
      director = models.ForeignKey('Director', on_delete=models.SET_NULL, null=True)
      year = models.IntegerField(null=True)
      genres = models.ManyToManyField(Genre)
  </code></pre>
  
  <p>I need to use movie_id as primary key, but also i need a field, which represents number of item's row in table.
  It must increment automatically, just like standard "id" field.
  How can i add it? </p>
  
  <p>This question <a href="https://stackoverflow.com/users/3404040/take-care">https://stackoverflow.com/users/3404040/take-care</a> is similar, but i can't use my "number" field as primary key, because movie_id is already used for that.</p>