檢查查詢是否是不存在的或具有特定值

Question

例如，我們有實例模型，我們想要做的事象下面這樣：

ex = Example.objects.filter(Q(participant__tries=0) | Q(participant__tries=*THERE IS NO EXAMPLE RECORD*), courses=course) 

我的意思是在數據庫中沒有記錄，不存在併爲空。還有一個查詢集從不返回任何對象。

我們應該在兩顆星之間取代什麼？

附錄：

我自己的模型

class Questions(models.Model): 
    course = models.ForeignKey(Course, blank=False, db_index=True) 
    question_author = models.ForeignKey(Author, blank=False) 
    number = models.IntegerField(default=0, editable=False, help_text="<b>***Please Don't change it as much as possible!</b>") 
    question_details = models.CharField(max_length=100, blank=False, default='') 
    members = models.ManyToManyField(User, through='QuestionsParticipant') 
    question_type = models.CharField(choices=question_types, max_length=1, default='1') 
    own_score = models.PositiveIntegerField(default=10) 
    approved = models.BooleanField(default=False) 
    timestamp = models.DateTimeField(auto_now_add=True) 

class QuestionsParticipant(models.Model): 
    question = models.ForeignKey(Questions) 
    participant = models.ForeignKey(User) 
    tries = models.PositiveIntegerField(blank=False, default=0) 
    flag = models.BooleanField(default=False) 
    flag_reason = models.CharField(max_length=50, blank=True, default='') 
    success = models.BooleanField(default=False) 
    timestamp = models.DateTimeField(auto_now_add=True) 

在我自己的看法：

question = Questions.objects.filter(Q(questionsparticipant__tries=0) | Q(questionsparticipant__tries__isnull=True), 
               Q(questionsparticipant__participant_id=request.user.id) 
               | Q(questionsparticipant__participant_id__isnull=True), 
               course=courses, approved=True).order_by("number")[0] 

Answer 1

取決於你的模型/架構是如何設置的。您可能需要提供更多詳細信息。

在很多情況下，isnull將工作。例如，Q(participant__tries__isnull=True)。請參閱official docs on isnull。