title: How to filter filter_horizontal in Django admin?
body: <p>I'm looking for a way to use filter_horizontal on the base of a filtered queryset.</p>
  
  <p>I've tried to use it with a custom manager:</p>
  
  <p><strong>In models.py:</strong></p>
  
  <pre><code>class AvailEquipManager(models.Manager):
      def get_query_set(self):
          return super(AvailEquipManager, self).get_query_set().filter(id=3)
  
  class Equipment(models.Model):
      description = models.CharField(max_length=50)
      manufacturer = models.ForeignKey(Manufacturer)
      [...]
      objects = models.Manager()
      avail = AvailEquipManager()
  
      def __unicode__(self):
          return u"%s" % (self.description)
  </code></pre>
  
  <p><strong>In admin.py:</strong></p>
  
  <pre><code>class SystemAdmin(admin.ModelAdmin):
      filter_horizontal = ('equipment',) # this works but obviously shows all entries
      #filter_horizontal = ('avail',)     # this does not work
  </code></pre>
  
  <p>So the questions is, how can I reduce the left side of the filter_horizontal to show only specific items?</p>