Primary key behavior in Django

Introduction

A primary key is a special field in a database table that uniquely identifies each record. It helps Django find and manage data easily.

When you want to uniquely identify each item in a database table.

When you need to link records between tables using relationships.

When you want to ensure no two records have the same identity.

When you want Django to automatically create unique IDs for your data.

Syntax

Django

class MyModel(models.Model):
    id = models.AutoField(primary_key=True)
    # other fields here

Django automatically adds an id field as a primary key if you don't define one.

You can set any field as primary key by adding primary_key=True.

Examples

This model uses the default id field as primary key.

Django

class Book(models.Model):
    title = models.CharField(max_length=100)
    # Django adds id automatically as primary key

Here, author_id is the primary key instead of the default id.

Django

class Author(models.Model):
    author_id = models.CharField(max_length=10, primary_key=True)
    name = models.CharField(max_length=100)

Using a custom string field as primary key.

Django

class Product(models.Model):
    sku = models.CharField(max_length=20, primary_key=True)
    name = models.CharField(max_length=100)

# Using a custom string field as primary key.

Sample Program

This example shows a model with a custom primary key student_number. We create and save a student, then retrieve it using the primary key.

Django

from django.db import models

class Student(models.Model):
    student_number = models.CharField(max_length=10, primary_key=True)
    name = models.CharField(max_length=100)

# Usage example (in Django shell or view):
# Create a student
student = Student(student_number='S12345', name='Alice')
student.save()

# Retrieve student by primary key
found_student = Student.objects.get(student_number='S12345')
print(f"Student: {found_student.name}, Number: {found_student.student_number}")

Important Notes

Primary key fields must be unique and not null.

Using AutoField lets Django auto-increment IDs for you.

Choosing a meaningful primary key can help with data clarity but must be unique.

Summary

Primary keys uniquely identify each record in a Django model.

Django adds an id field automatically if you don't specify one.

You can set any field as primary key by adding primary_key=True.