title: cannot assign django.contrib.auth.models.User to filed using south migration
body: <p>For my django1.4 app,I am trying to use south to do migration of data.I am using postgres8.3</p>
  
  <p>I have a <code>MyCategory</code> model for which I need to add a field called <code>creator</code> which is a django.contrib.auth.models.User</p>
  
  <p>I modified the class and added the field</p>
  
  <pre><code>from django.contrib.auth.models import User
  class MyCategory(models.Model):
      name=models.CharField(unique=True,max_length=50)
      description=models.TextField(blank=True)
      creator = models.ForeignKey(User,null=True)#added
      slug=models.SlugField(editable=False)
  </code></pre>
  
  <p>I ran <code>python manage.py schemamigration myapp --auto</code></p>
  
  <p>which output <code>+ Added field creator on myapp.MyCategory</code>
  and created a file <code>0003_auto__add_field_mycategory_creator.py</code></p>
  
  <p>Then I wanted all old records to have the superuser of the webapp as creator .So I tried datamigration</p>
  
  <p>I ran <code>python manage.py datamigration myapp add_creator</code></p>
  
  <p>This output a file add_creator.py in which I implemented the forwards method as below.(later I added a print stmt for debug purpose)</p>
  
  <pre><code>def forwards(self, orm):
      from django.contrib.auth.models import User
      suser = User.objects.filter(is_superuser=True)[0]
      # I also tried User.objects.get(is_superuser=True)
      print 'suser=',suser,'of type=',type(suser)
      for category in orm.MyCategory.objects.all():
          category.creator = suser
          category.save()
  </code></pre>
  
  <p>Then I ran the migrate command</p>
  
  <p><code>python manage.py migrate myapp</code></p>
  
  <p>This output a <code>ValueError</code></p>
  
  <pre><code>Running migrations for myapp:
   - Migrating forwards to 0004_add_creator.
   &gt; myapp:0004_add_creator
  suser= damon of type= &lt;class 'django.contrib.auth.models.User'&gt;
  Error in migration: myapp:0004_add_creator
  Traceback (most recent call last):
    File "manage.py", line 11, in &lt;module&gt;
      execute_manager(settings)
  ....
  File "/usr/local/lib/python2.6/dist-packages/south/migration/migrators.py", line 57, in &lt;lambda&gt;
      return (lambda: direction(orm))
    File "/home/me/dev/python/django/myapp/migrations/0004_add_creator.py", line 14, in forwards
      category.creator = suser
    File "/home/me/Django-1.4/django/db/models/fields/related.py", line 366, in __set__
      self.field.name, self.field.rel.to._meta.object_name))
  ValueError: Cannot assign "&lt;User: damon&gt;": "MyCategory.creator" must be a "User" instance.
  </code></pre>
  
  <p>I don't understand what is happening here..The print statement shows that the user <code>damon</code> is of type <code>django.contrib.auth.models.User</code> ..Is <code>south</code> expecting some other type? Can someone please help?</p>