Django:NotImplementedError:annotate()+distinct(字段)未实现

Question

有两个简单的模型：

class Question(TimeStampedModel):
    text = models.CharField(max_length=40)
class Answer(TimeStampedModel):
    question = models.ForeignKey(Question, related_name='answers')
    is_agreed = models.BooleanField()
    author = models.ForeingKey(User, related_name='answers')

还有我的问题：

In [18]: Question.objects.count()
Out[18]: 3

我需要用"is_user_agreed"one_answers"answers_amount"字段注释查询集：

In [18]: user = User.objects.first() 
In [19]: qs = Question.objects.annotate(
    ...:             is_user_agreed=Case(
    ...:                 When(answers__in=user.answers.filter(is_agreed=True), then=Value(True)),
    ...:                 When(answers__in=user.answers.filter(is_agreed=False), then=Value(False)),
    ...:                 default=Value(None),
    ...:                 output_field=NullBooleanField(),
    ...:             ),
    ...:         ).annotate(answers_amount=Count('answers'))
    ...: qs.count()
Out[19]: 4 

^这里的计数是4，但我在数据库中只有3个问题：(所以，我尝试了distinct()

In [20]: qs.distinct().count()
Out[20]: 4  # but distinct doesn't work
In [21]: qs.distinct('id').count()

在最后一行代码之后，我得到了一个异常：

NotImplementedError: annotate() + distinct(fields) is not implemented.

我也尝试过使用这个技巧annotate(Count('id')).filter(id__count__gt=1)

但在这种情况下，我将丢失所有重复的行，并且qs.count（）为2。

更新：问题是查询集中的行重复。

解决方案：（Vladimir第二种方法的扩展变体）

user = User.objects.first()
user_agreed_questions = user.answers.filter(
    is_agreed=True).values_list('question_id', flat=True)
user_not_agreed_questions = user.answers.filter(
    is_agreed=False).values_list('question_id', flat=True)
Question.objects.annotate(
    answer_amount=Count('answers'),
    is_user_agreed=Case(
         When(id__in=user_agreed_questions, then=True),
         When(id__in=user_not_agreed_questions, then=False),
         default=None,
         output_field=NullBooleanField(),
    ),
)

Answer 1

试试这个：

Question.objects.annotate(
    answer_amount=Count('answers'),
    is_user_agreed=F('answers__is_agreed'),
).order_by('id', '-answers__is_agreed').distinct('id')

如果question不具有answers，则question.is_user_agreed是None。如果question具有至少一个answer和answer.is_agreed=True，则question.is_agreed是True。否则，is_user_agreed就是False。

或者这个：

agreed_questons = Answer.objects.filter(
    is_agreed=True,
).values_list('question_id', flat=True).distinct()
Question.objects.annotate(
    answer_amount=Count('answers'),
    is_agreed=Case(
        When(id__in=agreed_questions, then=True),
        When(answers__isnull=True, then=None),
        default=False,
        output_field=NullBooleanField(),
    ),
)

agreed_questons是具有至少一个answer和answer.is_agreed=True的questions中的id的列表。