Autofill Django model fields when create instance of another model

Question

I have three models Student, Question and StudentAndQuestion

class Student(models.Model):
    class Meta:
        verbose_name_plural = "Students"
    name = models.CharField(max_length=150)
    surname = models.CharField(max_length=150)
    code = models.CharField(max_length=10)
#    group = models.ForeignKey(Group,on_delete=models.CASCADE)
    points = models.IntegerField(default=0)

class Question(models.Model):
    class Meta:
        verbose_name_plural = "Questions"

    text = models.CharField(max_length=1500)
    variants = models.CharField(max_length=1500)
    theme = models.OneToOneField(Theme, on_delete=models.CASCADE)
    anwser = models.CharField(max_length=1500)

class StudentAndQuestion(models.Model):
    question = models.OneToOneField(Question,on_delete=models.CASCADE)
    student = models.OneToOneField(Student,on_delete=models.CASCADE)
    is_learned = models.BooleanField(default=0)
    points = models.IntegerField(default=0)

I want Django make fill into StudentAndQuestion when i create new instance of students (for example create rows for all Questions and set points to zero) and when i create new instance of Question add it to all old students

are you sure of OneToOneField, this means when a studentAndQuestion instance has a question, other instances are no longer able to have it. ForeignKey is what you need to have it work + Django Signal — Lemayzeur
– Lemayzeur, Commented May 1, 2018 at 15:04

Resley Rodrigues · Accepted Answer · 2018-05-01 12:08:29Z

1

You can listen to the Post Save Signal emitted when these models are saved, and do the needed creation in the handler. This signal calls the handler with an argument called created which is True if a new record was created.

Refer to the documentation on signals to learn how to register your handlers. A sample is as follows

from django.db.models.signals import post_delete, post_save

from .models import Student, Question, StudentAndQuestion


def add_question_to_students(sender, instance, created, *args, **kwargs):    
question = instance
if created:
    StudentAndQuestion.objects.bulk_create([
        StudentAndQuestion(question=question, student_id=student_id)
        for student_id in Student.objects.values_list("id", flat=True)
    ])

def add_student_to_question(sender, instance, created, *args, **kwargs):    
student = instance
if created:
    StudentAndQuestion.objects.bulk_create([
        StudentAndQuestion(question=question_id, student=student)
        for question_id in Question.objects.values_list("id", flat=True)
    ])

post_save.connect(add_question_to_students, sender=Question)
post_save.connect(add_student_to_question, sender=Student)

Alternatively, you could override the save in both models to do the same

class Question(models.Model):
    ....
    def save(*args, **kwargs):
        is_create = self.pk is None
        instance = super().save(*args, **kwargs)
        add_question_to_students(Question, self, is_created)

class Student(models.Model):
    ....
    def save(*args, **kwargs):
        is_create = self.pk is None
        instance = super().save(*args, **kwargs)
        add_student_to_question(Student, self, is_created)

I feel the later is clearer as it is easier to find out what all is happening on save, but it can lead to circular dependencies if the models are in different apps.

answered May 1, 2018 at 12:08

Resley Rodrigues

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2 Comments

Danny Belchenko Over a year ago

I got "NOT NULL constraint failed" using second method

Resley Rodrigues Over a year ago

That's probably coz you did it before the super call... Or maybe self & instance are not equal, and it should have been self = super() but that feels weird

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