django 获取切片后无法过滤查询

Question

跟进：

AutoField should be present but it's not (django)?

相关：

"Cannot update a query once a slice has been taken". Best practices?

我不需要过滤对象，只按日期排序并选择最近的一个：

def get_latest_currency(self):
    """
    Return most up to date value
    """
    up_to_date_currency = Currency.objects.order_by('-currency_value_in_dollars_date')[:1]

    if not up_to_date_currency.exists():
        # No objects yet; fetch currencies
        update_coins_table()

    return Currency.objects.order_by('-currency_value_in_dollars_date')[:1]

up_to_date_currency 已正确初始化；最后一行给出：

    assert not self.query.is_sliced, \
AssertionError: Cannot filter a query once a slice has been taken.

这段代码代表一个视图（我想返回一个普通的 JSON 对象（这是一个 REST 端点））。为什么 django 在只使用切片时抱怨过滤器？

get_latest_currency 是端点的名称：

import sys

from django.urls import path
from . import views

app_name = 'manage_crypto_currency'
urlpatterns = [
    path('get_currency/', views.get_latest_currency, name='index'),
]

堆栈跟踪：

Internal Server Error: /get_currency/
Traceback (most recent call last):
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\core\handlers\exception.py", line 47, in inner
    response = get_response(request)
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\utils\deprecation.py", line 116, in __call__
    response = self.process_response(request, response)
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\middleware\clickjacking.py", line 26, in process_response
    if response.get('X-Frame-Options') is not None:
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\db\models\query.py", line 418, in get
    clone = self._chain() if self.query.combinator else self.filter(*args, **kwargs)
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\db\models\query.py", line 942, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "C:\projects\crypto-currency-board\venv\lib\site-packages\django\db\models\query.py", line 954, in _filter_or_exclude
    assert not self.query.is_sliced, \
AssertionError: Cannot filter a query once a slice has been taken.

尝试将对象包裹在 HttpReponse() 周围：

return HttpResponse(Currency.objects.order_by('-currency_value_in_dollars_date')[:1], content_type='application' '/json')

这可行，但由于某种原因，只返回了名称。

Answer 1

您正在返回 QuerySet 作为 view 的结果，但 view 不能返回 QuerySet。中间件旨在将其转换为 HTTP 响应，从而导致错误。

因此您的视图需要返回一些HttpResponse [Django-doc]。一个非常简单的响应是QuerySet 的str(…)：

def get_latest_currency(self):
    up_to_date_currency = Currency.objects.order_by('-currency_value_in_dollars_date')[:1]

    if not up_to_date_currency.exists():
        # No objects yet; fetch currencies
        update_coins_table()

    qs = Currency.objects.order_by('-currency_value_in_dollars_date')[:1]
    # …
    return HttpResponse(str(qs))

您当然可以返回更复杂的结果，例如通过渲染模板或返回序列化程序的结果。

例如如果你想序列化最后一个对象，你可以使用：

from django.http import JsonResponse
from django.core.serializers import serialize
from json import loads as jloads

def get_latest_currency(self):
    up_to_date_currency = Currency.objects.order_by('-currency_value_in_dollars_date')[:1]

    if not up_to_date_currency.exists():
        # No objects yet; fetch currencies
        update_coins_table()

    cr = Currency.objects.latest('-currency_value_in_dollars_date')
    data = serialize('json', [cr])
    return JsonResponse({'data' : jloads(data)})