Understanding Django Relationships: OneToOneField vs ForeignKey vs ManyToManyField

This content originally appeared on DEV Community and was authored by Vicente G. Reyes

A few hours ago, I was asked by a senior Django developer what the difference of OneToOneField, ForeignKey and ManyToManyField. I believe I gave a pretty good explanation about it so here I am writing about it for you.

When working with Django models, relationships between tables are essential to structure your data properly. Django provides three main types of relationships: OneToOneField, ForeignKey, and ManyToManyField. Knowing when and how to use them is key to building scalable and maintainable applications.

In this post, we’ll break them down with simple explanations and examples.

OneToOneField

A One-to-One relationship means that one record in a model is linked to exactly one record in another model.

Think of it as an extension of a model.

Example: A User profile where each user has exactly one profile.

from django.db import models
from django.contrib.auth.models import User

class Profile(models.Model):
    user = models.OneToOneField(User, on_delete=models.CASCADE)
    bio = models.TextField()
    birthdate = models.DateField()

• Each User can have only one Profile.

• If the User is deleted, the related Profile will also be deleted (CASCADE).

• Useful for adding extra information to a model without altering its original definition.

ForeignKey

A ForeignKey represents a One-to-Many relationship. One record in a model can be related to multiple records in another model.

Example: A blog where one Author can have many Posts.

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

class Post(models.Model):
    author = models.ForeignKey(Author, on_delete=models.CASCADE)
    title = models.CharField(max_length=200)
    content = models.TextField()

• One Author can write many Posts.

• Each Post belongs to exactly one Author.

• Deleting the Author deletes all their posts (because of CASCADE).

ManyToManyField

A Many-to-Many relationship means multiple records in one model can be related to multiple records in another model.

Example: A Student can enroll in many Courses, and each Course can have many Students.

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

class Course(models.Model):
    title = models.CharField(max_length=200)
    students = models.ManyToManyField(Student, related_name="courses")

• A student can be in multiple courses.

• A course can have multiple students.

• Django creates an intermediate table automatically to handle these relationships.

Quick Comparison

Relationship	Field Type	Example	Real-World Use Case
One-to-One	OneToOneField	User Profile	Extending user models, passports, IDs
One-to-Many	ForeignKey	Author → Post	Blogs, comments, categories
Many-to-Many	ManyToManyField	Student Course	Tags, memberships, playlists

Final Thoughts

• Use OneToOneField when each record should be uniquely linked to another record.

• Use ForeignKey when one object should be related to many others.

• Use ManyToManyField when both sides of the relationship can have multiple connections.

Understanding these fields helps you design cleaner, more efficient database schemas in Django—and prevents headaches down the road.

This content originally appeared on DEV Community and was authored by Vicente G. Reyes