title: django autoincrement based on foreignkey
body: <p>Let's say I have a model <code>Issue</code> that looks like this:</p>
  
  <pre><code>class Issue(models.Model):
      title = models.CharField(max_length=100)
      author = models.CharField(max_length=100)
      text = models.TextField()
  </code></pre>
  
  <p>And another one called <code>Comment</code> like so:</p>
  
  <pre><code>class Comment(models.Model):
      issue = models.ForeignKey(Issue)
      text = models.TextField()
      number = models.AutoField()
  </code></pre>
  
  <p>I want the comment number field to be relative to the issue.</p>
  
  <p>Which means that every issue will have its separate comment#1, comment#2, comment#3...etc.</p>
  
  <p>Can this be done in Django?</p>