Spatial queries don't work on inherited geometry fields

[jbronn]

In the following models TexasCity inherits from City:

from django.contrib.gis.db import models

class City(models.Model):
    name = models.CharField(max_length=30)
    point = models.PointField()
    objects = models.GeoManager()

    def __unicode__(self): return self.name

class TexasCity(City):
    tx_county = models.CharField(max_length=30)
    objects = models.GeoManager() # Why does this need to be explicit?

    def __unicode__(self):
        return '%s (%s County)' % (self.name, self.tx_county)

When performing the following query:

In [4]: qs = TexasCity.objects.distance(pnt)

This SQL is generated, which is incorrect (there is no "point" column on the TexasCity model):

In [23]: qs.query.as_sql()
Out[23]: 
(u'SELECT (ST_distance_sphere("tz_texascity"."point",%s)) AS "distance", "tz_city"."id", "tz_city"."name", "tz_city"."point", "tz_texascity"."city_ptr_id", "tz_texascity"."tx_county" FROM "tz_texascity" INNER JOIN "tz_city" ON ("tz_texascity"."city_ptr_id" = "tz_city"."id")',
 (<django.contrib.gis.db.backend.postgis.adaptor.PostGISAdaptor object at 0x319b610>,))