ValueError：无法查询“”：必须是“”实例答案

【问题标题】：ValueError: Cannot query "": Must be "" instanceValueError：无法查询“”：必须是“”实例
【发布时间】：2021-04-08 15:36:32
【问题描述】：

我有一个测验应用程序，用户可以根据需要多次尝试每个测验。我现在正试图在用户的个人资料上显示一些测验统计信息。我正在努力显示用户在测验中的平均分数。

首先，我得到正确答案的总数：

correct_answers = QuizTaker.objects.filter(user = self.user, completed = True).aggregate(Sum('correct_answers'))['correct_answers__sum']

这按预期工作。现在我需要得到每个测验的尝试次数并将其乘以每个测验中的问题数。然后我就可以计算平均分了。

我在尝试获取每个测验的尝试次数时遇到了困难。如果我硬编码如下，它可以工作：

number_attempts = QuizTaker.objects.filter(user = self.user, completed = True, quiz__title='Afrika').aggregate(Max('attempt_number'))['attempt_number__max']

但我需要每个测验的最大尝试次数。所以我需要做一个for loop。

quiztakers = QuizTaker.objects.filter(user = self.user, completed = True)
max_attempts = []
        for quiz in quiztakers:
            number_attempts = QuizTaker.objects.filter(user = self.user, completed = True, quiz=quiz).aggregate(Max('attempt_number'))['attempt_number__max']
            attempts.append(number_attempts)
        return max_attempts

但这给了我以下错误：

ValueError at /accounts/profile/
Cannot query "Username- Quiz: Afrika - Attempt number: 2": Must be "Quiz" instance.

相关型号：

class Quiz(models.Model):
    type_of_content = models.CharField(max_length=1, choices=CONTENT_TYPES)
    song = models.ForeignKey(Song, null=True, on_delete=models.CASCADE)
    title = models.CharField(max_length=15)
    slug = models.SlugField(blank=True)
    questions_count = models.IntegerField(default=0)

class QuizTaker(models.Model):
    user = models.ForeignKey(User, on_delete=models.CASCADE)
    quiz = models.ForeignKey(Quiz, on_delete=models.CASCADE)
    correct_answers = models.IntegerField(default=0)
    completed = models.BooleanField(default=False)
    attempt_number = models.PositiveIntegerField(default=0)
    timestamp = models.DateTimeField(auto_now_add=True)

请注意，attempt_number 是在 views 中计算的，并且按预期工作。

以及完整的追溯：

Environment:


Request Method: GET
Request URL: http://dev-lr:8000/accounts/profile/

Django Version: 3.0.3
Python Version: 3.8.2
Installed Applications:
['django.contrib.admin',
 'django.contrib.auth',
 'django.contrib.contenttypes',
 'django.contrib.sessions',
 'django.contrib.messages',
 'django.contrib.staticfiles',
 'django.contrib.humanize',
 'rest_framework',
 'bootstrap4',
 'bootstrapform',
 'languages',
 'django_countries',
 'import_export',
 'django_tables2',
 'django_filters',
 'nested_admin',
 'accounts',
 'vocab',
 'flash',
 'api',
 'videos',
 'quizzes',
 'django_cleanup.apps.CleanupConfig']
Installed Middleware:
['django.middleware.security.SecurityMiddleware',
 'django.contrib.sessions.middleware.SessionMiddleware',
 'django.middleware.common.CommonMiddleware',
 'django.middleware.csrf.CsrfViewMiddleware',
 'django.contrib.auth.middleware.AuthenticationMiddleware',
 'django.contrib.messages.middleware.MessageMiddleware',
 'django.middleware.clickjacking.XFrameOptionsMiddleware']


Template error:
In template C:\Users\mvren\OneDrive\Documents\Coding\Russki\mysite\templates\base.html, error at line 0
   Cannot query "Melissavr - Quiz: Afrika - Attempt number: 2": Must be "Quiz" instance.
   1 : <!DOCTYPE html>
   2 : {% load static %}
   3 : <html lang="en" dir="ltr">
   4 :   <head>
   5 :     <meta charset="utf-8">
   6 :     <meta name="viewport" content="width=device-width, initial-scale=1">
   7 :     <title>Linga Russki</title>
   8 : {% comment %} Bootstrap CSS {% endcomment %}
   9 :   <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.4.1/css/bootstrap.min.css" integrity="sha384-Vkoo8x4CGsO3+Hhxv8T/Q5PaXtkKtu6ug5TOeNV6gBiFeWPGFN9MuhOf23Q9Ifjh" crossorigin="anonymous">
   10 : {% comment %} Google fonts {% endcomment %}


Traceback (most recent call last):
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\core\handlers\exception.py", line 34, in inner
    response = get_response(request)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\core\handlers\base.py", line 115, in _get_response
    response = self.process_exception_by_middleware(e, request)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\core\handlers\base.py", line 113, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "C:\Users\mvren\OneDrive\Documents\Coding\Russki\mysite\accounts\views.py", line 49, in profile
    return render(request, 'accounts/profile.html', context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\shortcuts.py", line 19, in render
    content = loader.render_to_string(template_name, context, request, using=using)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\loader.py", line 62, in render_to_string
    return template.render(context, request)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\backends\django.py", line 61, in render
    return self.template.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 171, in render
    return self._render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 163, in _render
    return self.nodelist.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 936, in render
    bit = node.render_annotated(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 903, in render_annotated
    return self.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\loader_tags.py", line 150, in render
    return compiled_parent._render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 163, in _render
    return self.nodelist.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 936, in render
    bit = node.render_annotated(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 903, in render_annotated
    return self.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\loader_tags.py", line 62, in render
    result = block.nodelist.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 936, in render
    bit = node.render_annotated(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 903, in render_annotated
    return self.render(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 986, in render
    output = self.filter_expression.resolve(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 670, in resolve
    obj = self.var.resolve(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 795, in resolve
    value = self._resolve_lookup(context)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\template\base.py", line 857, in _resolve_lookup
    current = current()
  File "C:\Users\mvren\OneDrive\Documents\Coding\Russki\mysite\accounts\models.py", line 65, in quiz_stats
    number_attempts = QuizTaker.objects.filter(user = self.user, completed = True, quiz=quiz).aggregate(Max('attempt_number'))['attempt_number__max']
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\manager.py", line 82, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\query.py", line 904, in filter
    return self._filter_or_exclude(False, *args, **kwargs)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\query.py", line 923, in _filter_or_exclude
    clone.query.add_q(Q(*args, **kwargs))
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\sql\query.py", line 1350, in add_q
    clause, _ = self._add_q(q_object, self.used_aliases)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\sql\query.py", line 1377, in _add_q
    child_clause, needed_inner = self.build_filter(
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\sql\query.py", line 1284, in build_filter
    self.check_related_objects(join_info.final_field, value, join_info.opts)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\sql\query.py", line 1122, in check_related_objects
    self.check_query_object_type(value, opts, field)
  File "C:\Users\mvren\miniconda3\envs\myRuEnv\lib\site-packages\django\db\models\sql\query.py", line 1103, in check_query_object_type
    raise ValueError(

Exception Type: ValueError at /accounts/profile/
Exception Value: Cannot query "Melissavr - Quiz: Afrika - Attempt number: 2": Must be "Quiz" instance.

【问题讨论】：

标签： python django django-models valueerror

【解决方案1】：

这不起作用，因为您尝试通过 Quiztaker 实例查询，您应该通过 Quiz 实例查询

    for quiztaker in quiztakers:
        number_attempts = QuizTaker.objects.filter(user=self.user, completed=True, 
            quiz=quiztaker.quiz).aggregate(Max('attempt_number')) 
            ['attempt_number__max']
        attempts.append(number_attempts)
    return max_attempts

这也不是最好的主意，因为您可以通过使用 annotate 和 values（GROUP BY 语句）进行适当的查询来优化它

【讨论】：