title: Configuring Many-to-many field in django admin with related_name
body: <p>I have the following model and admin defined in djando 1.5. This is a many-to-many relationship between subnet and vlan. I use the related_name option in the ManyToMany field to be able to get the collection of vlan from the related subnet object.  Adding subnet to vlans from the admin works well.  However when I try to add an horizontal_filer to the subnet admin in order to add van to its vlan set I get an error saying that the vlans attribute doesn't exist. I'm using subnet object in some view and I can access the vlans attribute just right.</p>
  
  <p>What am I doing wrong here ?  I've seen similar post but I couldn't adapt any of the proposed solution with success.</p>
  
  <p>Thanks for your help</p>
  
  <p>model.py</p>
  
  <pre><code>from django.db import models
  
  class Subnet(models.Model):
      networkAddress = models.CharField(max_length=15)
      size = models.IntegerField()
  
      def __unicode__(self):
          return "%s/%s" % (self.networkAddress, self.size)
  
  class IpAddress(models.Model):
      ipAddress = models.CharField(max_length=15)
      subnet = models.ForeignKey(Subnet)
  
      def __unicode__(self):
          return "%s" % (self.ipAddress)
  
  class Vlan(models.Model):
      number = models.IntegerField()
      description = models.CharField(max_length=150)
      subnets = models.ManyToManyField(Subnet, related_name='vlans', blank=True)
  
      def __unicode__(self):
          return "VLAN %s (%s)" % (self.number, self.description)
  </code></pre>
  
  <p>admin.py </p>
  
  <pre><code>from network.models import Subnet, IpAddress, Vlan
  from django.contrib import admin
  
  class SubnetAdmin(admin.ModelAdmin):
      filter_horizontal = ('vlans',)
  
  admin.site.register(Subnet, SubnetAdmin)
  admin.site.register(IpAddress)
  admin.site.register(Vlan)
  </code></pre>
  
  <p>and the error I get</p>
  
  <pre><code>Request Method:     GET
  Request URL:    http://127.0.0.1:8000/admin/
  Django Version:     1.5.2
  Exception Type:     ImproperlyConfigured
  Exception Value:    
  
  'SubnetAdmin.filter_horizontal' refers to field 'vlans' that is missing from model 'network.Subnet'.
  </code></pre>