django admin.py 外键,反向查询的实例

class OrderAdmin(admin.ModelAdmin):  list_display = (    '_nick_name', 'time_order', 'year', 'item', 'status', 'number',    'money', 'deduction_point', 'deduction_account', 'pay',    '_open_id', 'out_trade_no', )  search_fields = [    'user__nick_name', 'user__wechatuser__open_id',    'out_trade_no']  list_filter = ('year', 'item', 'status')  def _nick_name(self, obj):    return obj.user.nick_name  def _open_id(self, obj):    return obj.user.wechatuser.open_id