一个生成django queryset.filter参数的例子。核心内容：只需按照常规的filter写法将参数和值拆分为一个tuple即可。如，qs.filter(id=5)对应为(id,5)。如需复杂的条件，需要继承自django.utils.tree.Node。

expressions.py

MODE_PROVINCE = 10000
MODE_CITY = 100


def D(district_field_name, district_id):
    '''
    获取区域选筛选表达式
    @param district_field_name:
    @param district_id:
    @return:(字段表达式,值)
    '''
    assert isinstance(district_id, int)

    if district_id % MODE_PROVINCE == 0:  # 省级单位
        return '%s__range' % district_field_name, (district_id, district_id + MODE_PROVINCE)
    if district_id % MODE_CITY == 0:  # 市级单位
        return '%s__range' % district_field_name, (district_id, district_id + MODE_CITY)
    else:
        return district_field_name, district_id