django – 如何使地理领域独一无二？

class Marker(models.Model):
    location = models.PointField(geography=True,unique=True)

当我尝试通过管理界面添加标记实例时,它会引发一个ValueError：

File "django/core/handlers/base.py" in get_response
  111. response = callback(request,*callback_args,**callback_kwargs)
File "django/contrib/admin/options.py" in wrapper
  366. return self.admin_site.admin_view(view)(*args,**kwargs)
File "django/utils/decorators.py" in _wrapped_view
  91. response = view_func(request,*args,**kwargs)
File "django/views/decorators/cache.py" in _wrapped_view_func
  89. response = view_func(request,**kwargs)
File "django/contrib/admin/sites.py" in inner
  196. return view(request,**kwargs)
File "django/utils/decorators.py" in _wrapper
  25. return bound_func(*args,**kwargs)
File "django/utils/decorators.py" in bound_func
  21. return func(self,*args2,**kwargs2)
File "django/db/transaction.py" in inner
  209. return func(*args,**kwargs)
File "django/contrib/admin/options.py" in add_view
  937. if form.is_valid():
File "django/forms/forms.py" in is_valid
  124. return self.is_bound and not bool(self.errors)
File "django/forms/forms.py" in _get_errors
  115. self.full_clean()
File "django/forms/forms.py" in full_clean
  272. self._post_clean()
File "django/forms/models.py" in _post_clean
  338. self.validate_unique()
File "django/forms/models.py" in validate_unique
  347. self.instance.validate_unique(exclude=exclude)
File "django/db/models/base.py" in validate_unique
  633. errors = self._perform_unique_checks(unique_checks)
File "django/db/models/base.py" in _perform_unique_checks
  724. if qs.exists():
File "django/db/models/query.py" in exists
  565. return self.query.has_results(using=self.db)
File "django/db/models/sql/query.py" in has_results
  441. return bool(compiler.execute_sql(SINGLE))
File "django/db/models/sql/compiler.py" in execute_sql
  808. sql,params = self.as_sql()
File "django/db/models/sql/compiler.py" in as_sql
  82. where,w_params = self.query.where.as_sql(qn=qn,connection=self.connection)
File "django/db/models/sql/where.py" in as_sql
  91. sql,params = child.as_sql(qn=qn,connection=connection)
File "django/db/models/sql/where.py" in as_sql
  94. sql,params = self.make_atom(child,qn,connection)
File "django/contrib/gis/db/models/sql/where.py" in make_atom
  47. spatial_sql = connection.ops.spatial_lookup_sql(data,lookup_type,params_or_value,lvalue.field,qn)
File "django/contrib/gis/db/backends/postgis/operations.py" in spatial_lookup_sql
  497. '"%s" lookup.' % lookup_type)

Exception Type: ValueError at /admin/coremap/marker/add/
Exception Value: PostGIS geography does not support the "exact" lookup.

根据this,地理类型确实没有确切的字段查找.有没有办法在不使用精确的情况下完成唯一约束？

我正在使用：

> Postgresql 9.1
> PostGIS 1.5
> Django 1.4.3