more than 1 foreign key

I have the following models: http://slexy.org/view/s20T8yOiKZ

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("开发者_如何学GoCategory_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',)

When I try to click add button for worlds in admin page it shows me the following error:

class 'cms_sample.world_models.Element' has more than 1 ForeignKey to class 'cms_sample.world_models.World'.

I think it's something to do with inline. What can it be?

Django doesn't know which of the two foreign keys (parent and world) is to be inlined using the ElementInline.

class ElementInline(admin.TabularInline):
    model = Element
    fk_name = 'parent' #or 'world', as applicable.
    extra=1