How to save multiple ManytoMany instances to a Django object?

问题

I have a django app with several Question objects. Question and Quiz are related through a ManyToManyField. I want to create Quiz objects such that each quiz is randomly assigned 3 questions.

I've tried to create a save method in my model where I: use a queryset to get 3 questions, save the Quiz instance (to save an id), set the queryset to the Quiz and save again. Most of my searches on this topic have shown where people are using a form to save the m2m object.

Even better, is it possible to set 3 questions to a quiz object using a view? Ultimately this is where I want to get to. A user will select a LearnGoal using request.GET in a view, and this LearnGoal will restrict the questions that will be added to the Quiz.

My models.py so far:

class LearnGoal(models.Model):
    goal_name = models.CharField(max_length=12)
    goal_description = models.TextField()

class Question(models.Model):
    name = models.CharField(max_length=12)
    learngoal = models.ForeignKey(LearnGoal, on_delete=models.CASCADE)
    q_text = models.TextField()
    answer = models.CharField(max_length=12)

class Quiz(models.Model):
    """quiz which will have three questions."""
    name = models.CharField(max_length=12)
    questions = models.ManyToManyField(Question)
    completed = models.DateTimeField(auto_now_add=True)
    my_answer = models.CharField(max_length=12)

    def save(self, *args, **kwargs):
        """need to create an instance first, for the m2m"""
        super(Quiz, self).save(*args, **kwargs)
        """get 3 random questions"""
        three_questions = Quiz.objects.all().order_by('?')[0:3]
        self.questions.set(three_questions)
        super(Quiz, self).save(*args, **kwargs)

I've tried a few variations of the above code. Currently this code gives the error: 'Question' instance expected, got <Quiz: Quiz object (4)> from the line self.questions.set(five_questions)

How can I save multiple questions to one instance of the quiz?

回答1:

In the Quiz.save method, you can do:

class Quiz(models.Model):
    ...
    ...

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.add_questions = False

    def save(self, *args, **kwargs):
        # We need this to prevent updating questions
        # on updating the Quiz instance
        if self.pk is None:
            self.add_questions = True
        super().save(*args, **kwargs)

        if self.add_questions:
            # No need to run save again; M2M relationships are set
            # through an intermediate model
            self.questions.set(Question.objects.order_by('?')[:3])

Note that, random ordering (order_by('?')) is slow but whether that will affect you totally depends on the use case.

来源：https://stackoverflow.com/questions/58787050/how-to-save-multiple-manytomany-instances-to-a-django-object

标签

django

manytomanyfield