title: Django: "limit_choices_to" doesn't work on ManyToManyField
body: <p>I am running Django 1.1 and cannot get the "limit_choices_to" option for my ManytoManyField to work.</p>
  
  <p>I have two models:</p>
  
  <pre><code>class MemberPhoto(ImageModel):
      title       = models.CharField(_('title'), max_length=255, blank=True, null=True)
      caption     = models.CharField(_('caption'), max_length=255, blank=True, null=True)
      date_added  = models.DateTimeField(_('date added'), default=datetime.now, editable=False)
      member      = models.ForeignKey(User)
  
      def __unicode__(self):
          return u'%s (%s)' % (self.member.username, self.id)
  </code></pre>
  
  <p>and</p>
  
  <pre><code>class lock(models.Model):
      user = models.ForeignKey(User, related_name="owner")
      to_user = models.ForeignKey(User, related_name="to_user")
      unlocked_photos = models.ManyToManyField(MemberPhoto, blank=True, null=True, limit_choices_to = {'member':'user'})
      objects = locking_manager()
  </code></pre>
  
  <p>in the second model, i want to make sure in Django's admin that the only "unlocked_photos" ("MemberPhoto" objects) presented in the multiple select field are those who have a "member" value (a User object) the same as the "lock" object's "user" (also a User object).</p>
  
  <p>I thought I had followed the Django docs on this, but it doesn't work. I get the following error:</p>
  
  <pre><code>TemplateSyntaxError
  
  Caught an exception while rendering: invalid input syntax for integer: "user"
  </code></pre>
  
  <p>I've tried changing the "limit_choices_to" to things like:</p>
  
  <p>limit_choices_to = {'member': user} --- Doesnt work</p>
  
  <p>limit_choices_to = {'member__username':'kyle'} --- this DOES work but it's useless, i'm just manually specifying a username</p>
  
  <p>How can I instead get the user from the current "lock" object and filter the MemberPhoto "member" property by that?</p>
  
  <p>Thanks to anybody who can help.</p>
  
  <p>Kyle</p>