Django 超过 1 个外键

我有以下型号: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("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',)

当我try 单击管理页面中世界的添加按钮时，它显示以下错误:

类"cms_sample.world_mods.Element"具有多个指向类"cms_sample.world_mods.World"的ForeignKey.

我认为这与内联有关.