title: OperationalError1075 'Incorrect table definition'
body: <p>I changed one of my models to go from </p>
  
  <pre><code>serial_number = models.BigIntegerField(unique=True)
  </code></pre>
  
  <p>to </p>
  
  <pre><code>serial_number = models.AutoField(primary_key=True)
  </code></pre>
  
  <p>and ran </p>
  
  <pre><code>$ python manage.py check -v 3
  $ python manage.py makemigrations -v 2
  $ python manage.py migrate -v 2
  </code></pre>
  
  <p>However, when I try to run the unit tests (which passed fine before), I get this error:</p>
  
  <pre><code>[...]
    File "/home/usr/Envs/intranet/lib/python2.7/site-packages/MySQLdb/connections.py", line 36, in defaulterrorhandler
      raise errorclass, errorvalue django.db.utils.OperationalError: (1075, 'Incorrect table definition; there can be only one auto column and it must be defined as a key')
  </code></pre>
  
  <p><em>How to resolve this?</em></p>