title: IntegrityError: null value in column "city_id " violates not-null constraint
body: <p>I two model:</p>
  
  <pre><code>class City(models.Model):
      name = models.CharField(max_length=50)
      country = models.OneToOneField(Country)
  
      def __unicode__(self):
          return self.name 
  
  
  class UserProfile(models.Model):
      user = models.OneToOneField(User)
      city = models.OneToOneField(City)
  </code></pre>
  
  <p>when I syncdb and create admin user :</p>
  
  <pre><code>IntegrityError: null value in column "city_id" violates not-null constraint
  </code></pre>
  
  <p>How I can Fix this error?</p>