title: Django MTMField: limit_choices_to = other_ForeignKeyField_on_same_model?
body: <p>I've got a couple django models that look like this:</p>
  
  <pre><code>from django.contrib.sites.models import Site
  
  class Photo(models.Model):
      title = models.CharField(max_length=100)
      site = models.ForeignKey(Site)
      file = models.ImageField(upload_to=get_site_profile_path) 
  
      def __unicode__(self):
          return self.title
  
  
  class Gallery(models.Model):    
      name = models.CharField(max_length=40)
      site = models.ForeignKey(Site)
      photos = models.ManyToManyField(Photo, limit_choices_to = {'site':name} )    
  
      def __unicode__(self):
          return self.name
  </code></pre>
  
  <p>I'm having all kinds of <em>fun</em> trying to get the <code>limit_choices_to</code> working on the Gallery model.  I only want the Admin to show choices for photos that belong to the same site as this gallery.  Is this possible?</p>