title: more than 1 foreign key
body: <p>I have the following models: <a href="http://slexy.org/view/s20T8yOiKZ" rel="noreferrer">http://slexy.org/view/s20T8yOiKZ</a></p>
  
  <pre><code>from mxutils.cms_services import generate_secid
  from django.db import models
  from django.contrib import admin
  from django import forms
  
  class World(models.Model):
      title = models.CharField(max_length=150)
      secid = models.SlugField(max_length=1000, editable=False)
      elements = models.ManyToManyField("Element", related_name='elements', blank=True, null=True)
      metadata = models.OneToOneField("Category_metadata", blank=True, null=True)
      def save(self):
          if not self.pk:
              super(World, self).save()
              self.secid = generate_secid(self.title, self.pk, World.objects.all())
          return super(World, self).save()
      def __unicode__(self):
          return "%s" % self.title
  
  class Element(models.Model):
      parent = models.ForeignKey(World, related_name='element_parent')
      world = models.ForeignKey(World, related_name='world', blank=True, null=True)
      item = models.ForeignKey("Item", blank=True, null=True)
      value = models.DecimalField(default=0, max_digits=5, decimal_places=3)
      def save(self):
          if self.world and self.item:
              return None
          elif not self.world and not self.item:
              return None
          else:
              return super(Element, self).save()
      def __unicode__(self):
          if self.world:
              return "%s" % self.world.title
          else:
              return "%s" % self.item.title
  
  class ElementInline(admin.TabularInline):
      model = Element
      extra=1
  
  class WorldAdmin(admin.ModelAdmin):
      inlines = [ElementInline,]
      list_display = ('title',)
      ordering = ['title']
      search_fields = ('title',)
  </code></pre>
  
  <p>When I try to click add button for worlds in admin page it shows me the following error: </p>
  
  <blockquote>
    <p>class 'cms_sample.world_models.Element' has more than 1 ForeignKey to class 'cms_sample.world_models.World'.   </p>
  </blockquote>
  
  <p>I think it's something to do with inline. 
  What can it be?</p>