It is about django related manager

Django 5.1 What is the meaning of q.choice_set.all() in the polls app? https://docs.djangoproject.com/en/5.1/intro/tutorial04/

from django.db.models import F
from django.http import HttpResponse, HttpResponseRedirect
from django.shortcuts import get_object_or_404, render
from django.urls import reverse

from .models import Choice, Question


def vote(request, question_id):
     question = get_object_or_404(Question, pk=question_id)
try:
    selected_choice = question.choice_set.get(pk=request.POST["choice"])
except (KeyError, Choice.DoesNotExist):
    # Redisplay the question voting form.
    return render(
        request,
        "polls/detail.html",
        {
            "question": question,
            "error_message": "You didn't select a choice.",
        },
    )
else:
    selected_choice.votes = F("votes") + 1
    selected_choice.save()
    # Always return an HttpResponseRedirect after successfully dealing
    # with POST data. This prevents data from being posted twice if a
    # user hits the Back button.
    return HttpResponseRedirect(reverse("polls:results", args=(question.id,)))

This is described in the Playing with the API section of the docs.

To better understand this, create your model classes:

from django.db import models


class Question(models.Model):
    question_text = models.CharField(max_length=200)
    pub_date = models.DateTimeField("date published")


class Choice(models.Model):
    question = models.ForeignKey(Question, on_delete=models.CASCADE)
    choice_text = models.CharField(max_length=200)
    votes = models.IntegerField(default=0)

Apply your migrations and then invoke the Python shell using python manage.py shell

>>> from polls.models import Choice, Question  # Import the model classes we just wrote.
>>> from django.utils import timezone

# Create a new Question.
>>> q = Question(question_text="What's new?", pub_date=timezone.now()) 

# Save the object into the database. You have to call save() explicitly.

>>> q.save()

From the docs:

Give the Question a couple of Choices. The create call constructs a new Choice object, does the INSERT statement, adds the choice to the set of available choices and returns the new Choice object. Django creates a set (defined as "choice_set") to hold the "other side" of a ForeignKey relation (e.g. a question's choice) which can be accessed via the API.

>>> q = Question.objects.get(pk=1)

# Display any choices from the related object set -- none so far.
>>> q.choice_set.all()
<QuerySet []>
# Create three choices.
>>> q.choice_set.create(choice_text="Not much", votes=0)
<Choice: Not much>
>>> q.choice_set.create(choice_text="The sky", votes=0)
<Choice: The sky>
>>> c = q.choice_set.create(choice_text="Just hacking again", votes=0)

# Choice objects have API access to their related Question objects.
>>> c.question
<Question: What's up?>

# And vice versa: Question objects get access to Choice objects.
>>> q.choice_set.all()
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>
>>> q.choice_set.count()
3

# The API automatically follows relationships as far as you need.
# Use double underscores to separate relationships.
# This works as many levels deep as you want; there's no limit.
# Find all Choices for any question whose pub_date is in this year
# (reusing the 'current_year' variable we created above).
>>> Choice.objects.filter(question__pub_date__year=current_year)
<QuerySet [<Choice: Not much>, <Choice: The sky>, <Choice: Just hacking again>]>

# Let's delete one of the choices. Use delete() for that.
>>> c = q.choice_set.filter(choice_text__startswith="Just hacking")
>>> c.delete()

See Playing with the API.

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