How to Use a Foreign Key to Create Many-to-One Relationships in Django
In Django, there are three main types of relationships: one-to-one, many-to-one, and many-to-many.
In this article, I will explain the many-to-one relationship in Django. If you are a beginner with some knowledge about Django project setup or even have some decent experience in Django, you can follow along with this article.
Many-to-one relationship are sometimes referred to as one-to-many relationships. As you'll see below, these are related terms.
What is a Many-to-One Relationship?
Many-to-one relationship is a type of relationship where multiple records in one table are associated to the single record in another table.
Let's assume we have two tables in a database:
Employee. The relationship between
Employee is a
one-to-many relationship. A
department can have many
employees and each
employee belongs to one
And, the relationship between the
departments is a
This can be illustrated with the diagram below:
Diagram to illustrate many-to-one relationship
For example, an
Account department can have many
employees (one-to-many) and those
employees all belong to the
account department (many-to-one).
How to Create Models for Many-to-one Relationships
For this tutorial, the app will have two models:
So let's start adding code for the models.
from django.db import models class Department(models.Model): name=models.CharField(max_length=50) def __str__(self): return self.name class Employee(models.Model): name=models.CharField(max_length=70) address=models.CharField(max_length=90) department=models.ForeignKey(Department, on_delete=models.CASCADE) def __str__(self): return self.name
Department model contains only one field,
Employee model contains
address fields, and also a
ForeignKey field (
department) which references the
Department model. That's the reason it's passing
Department as the first argument.
The second argument,
on_delete, specifies the behavior to adopt when the reference object is deleted, which is
Department in this case. The
models.CASCADE option means that if a
Department object is deleted, all related
Employee objects will also be deleted.
What is a Foreign Key in a Django Model?
The foreign key is used to connect two tables and establishes the
many-to-one relationship. You can define a
foreign key in a Django model using the
models.ForeignKey field. And this
ForeignKey field takes at least two arguments:
department = models.ForeignKey(Department, on_delete=models.CASCADE)
We already discussed these two arguments in the above section.
And you can also pass other arguments like
related_name with a value which lets you access
foreign keys defined in your Django models backwards.
Now, you can make the migrations using the
And then you can make changes to the database using the
How to Interact with Models
Now, let's play around with shell to understand the primary concepts of
First, you need to run the