Django: objects and model_set

Question

I am learning django 1.10 official tutorial part 2

class  Question(models.Model):
  # ......

class Choice(models.Model):
  question = models.ForeignKey(Question, on_delete=models.CASCADE)
  #.......

Recent i saw the following command:-

q = Question.objects.get(id=1)
q.choice_set.all()

My questions:-

How Question instance contain choice_set, i know it for accessing related objects.

Why is this not valid

c = Choice.objects.get(id=1)
c.question_set.all()

Answer 1

The Question model does not have an explicit reference to the Choice model; however, Django automatically adds a reverse-reference, which is by default called choice_set. You can override this by related_name keyword on the model such as:

class Choice(models.Model):
  question = models.ForeignKey(Question, on_delete=models.CASCADE, related_name='choices')

Now you can reference all of the choices for a question like this:

q = Question.objects.get(pk=1)
q.choices.all()

To answer your second question, the reason you cannot use reference question_set.all() from a choice object is because for each choice, there is only one question plus there is an explicit reference to the question object. In other words the Choice model already has a field called Question, which points to the Question model.

Hope that helps.