One-to-Many Relationships

Modeling One-to-Many

One-to-many means one row on one side can relate to many rows on the other side — implemented by putting a ForeignKey on the many model.

Example: Modeling One-to-Many

One Chapter can be the parent of many Tutorial rows, each pointing back to it through the chapter field.

from django.db import models

class Chapter(models.Model):
    title = models.CharField(max_length=200)

class Tutorial(models.Model):
    chapter = models.ForeignKey(
        Chapter, related_name='tutorials', on_delete=models.CASCADE
    )
    title = models.CharField(max_length=200)

Looking Up Children from the Parent

related_name gives you a friendly way to go from the one side back to all its related many rows.

Note: Without related_name, Django defaults to modelname_set, like chapter.tutorial_set.all().

Example: Looking Up Children from the Parent

Because related_name=tutorials was set on the ForeignKey, chapter.tutorials.all() returns every Tutorial linked to this chapter.

from core.models import Chapter

chapter = Chapter.objects.first()
chapter.tutorials.all()

on_delete Behavior

on_delete tells Django what to do to child rows when their parent is deleted — CASCADE deletes them too, SET_NULL clears the link instead.

Warning: Choosing CASCADE by default can silently delete more data than intended.

Example: on_delete Behavior

If the related Chapter is deleted, this Tutorial's chapter field becomes null instead of the Tutorial itself being deleted.

from django.db import models

class Tutorial(models.Model):
    chapter = models.ForeignKey(
        'Chapter', null=True, on_delete=models.SET_NULL
    )