How do Django model relationships work (ForeignKey, ManyToMany, OneToOne)?

Expected Interview Answer

Step-by-Step Explanation

1. Step 1

   Pick the relationship type

   Choose ForeignKey for many-to-one, ManyToManyField for many-to-many, and OneToOneField for a unique one-to-one link.

2. Step 2

   Declare the field with a target

   Point the field at the related model, using a string name for models defined later or in another app.

3. Step 3

   Set on_delete for keyed fields

   ForeignKey and OneToOneField require on_delete, such as CASCADE, PROTECT, or SET_NULL, to define delete behaviour.

4. Step 4

   Add related_name for reverse access

   Name the reverse accessor so traversing from the other side is readable and avoids clashes.

5. Step 5

   Migrate and query efficiently

   Run makemigrations and migrate, then use select_related for FK/O2O and prefetch_related for M2M to avoid N+1 queries.

Code Example

from django.db import models

class Author(models.Model):
    name = models.CharField(max_length=100)

class Profile(models.Model):
    # OneToOne: one profile per author
    author = models.OneToOneField(Author, on_delete=models.CASCADE, related_name='profile')
    bio = models.TextField()

class Book(models.Model):
    # ForeignKey: many books, one author
    author = models.ForeignKey(Author, on_delete=models.CASCADE, related_name='books')
    title = models.CharField(max_length=200)
    # ManyToMany: a book has many tags, a tag many books
    tags = models.ManyToManyField('Tag', related_name='books')

class Tag(models.Model):
    name = models.CharField(max_length=50)

# Traversing relationships
author = Author.objects.select_related('profile').get(pk=1)
author.books.all()          # reverse ForeignKey
author.profile.bio          # reverse OneToOne
book = Book.objects.prefetch_related('tags').first()

Follow-up Questions

• What does the on_delete argument control and what options exist?
• How do you add extra fields to a ManyToMany relationship?
• What is the difference between select_related and prefetch_related?
• When would you use a OneToOneField instead of a ForeignKey?
• How does related_name change reverse queries?