我想通过串联字符串过滤某些数据库对象。

正常的SQL查询为：

SELECT concat(firstName, ' ', name) FROM person WHERE CONCAT(firstName, ' ', name) LIKE "a%";

在模型中，我创建了一个名为PersonObjects的管理器：

class PersonObjects(Manager):
    attrs = { 
        'fullName': "CONCAT(firstName, ' ', name)"
    }

    def get_query_set(self):
        return super(PersonObjects, self).get_query_set().extra(
            select=self.attrs)

我也在我的模型中配置了这个：

objects = managers.PersonObjects()

现在，访问fullName可用于单个对象：

>>> p = models.Person.objects.get(pk=4)
>>> p.fullName
u'Fred Borminski'

但这在过滤器中不起作用：

>>> p = models.Person.objects.filter(fullName__startswith='Alexei')
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/usr/lib/python2.7/site-packages/django/db/models/manager.py", line 141, in filter
    return self.get_query_set().filter(*args, **kwargs)
  File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 550, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "/usr/lib/python2.7/site-packages/django/db/models/query.py", line 568, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1128, in add_q
    can_reuse=used_aliases)
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1026, in add_filter
    negate=negate, process_extras=process_extras)
  File "/usr/lib/python2.7/site-packages/django/db/models/sql/query.py", line 1191, in setup_joins
    "Choices are: %s" % (name, ", ".join(names)))
FieldError: Cannot resolve keyword 'fullName' into field. Choices are: firstName, gender, name, (...)

这是错误还是功能？我怎样才能解决这个问题？

谢谢。