Use key_name instead of link_id
Some entities do not have a link_id, but all entities are guaranteed
to have a key_name (or an id).
from django.contrib.gis.db import models
from django.contrib.localflavor.us.models import USStateField
class Location(models.Model):
name = models.CharField(max_length=50)
point = models.PointField()
objects = models.GeoManager()
class City(models.Model):
name = models.CharField(max_length=50)
state = USStateField()
location = models.ForeignKey(Location)
objects = models.GeoManager()