title: Django subclass model gives "duplicate key value violates unique constraint" on save
body: <p>I have a class with many fields:</p>
  <pre><code>class Parent(models.Model):
      id = models.AutoField(primary_key=True)
      ... many more fields
  </code></pre>
  <p>and I create a subclass</p>
  <pre><code>class Child(Parent):
      other_field = models.CharField(max_length=512, blank=True, null=True)
      date_featured = models.DateField()
  </code></pre>
  <p>After I migrate and create a Child object in the admin I get</p>
  <pre><code>duplicate key value violates unique constraint &quot;parent_pkey&quot;
  DETAIL:  Key (id)=(5) already exists.
  </code></pre>
  <p>I've seen some similar questions that suggest that you modify the database but I can't easily do that.  Do I need to change the id of the subclass?</p>