Django Models

A model is a Python class that describes one table in your database. Django creates the table for you.

The Database

A new project uses SQLite by default. The whole database is one file, db.sqlite3, so there is nothing to install. Later you can switch to PostgreSQL or MySQL by changing DATABASES in settings.py, without changing your models.

Your First Model

Edit courses/models.py:

Example

python

from django.db import models

class Course(models.Model):
    name = models.CharField(max_length=100)
    fee = models.PositiveIntegerField()
    duration_months = models.PositiveSmallIntegerField(default=3)
    is_online = models.BooleanField(default=False)
    description = models.TextField(blank=True)
    created_at = models.DateTimeField(auto_now_add=True)

    def __str__(self):
        return self.name

Each attribute becomes a column
Django adds an id column (the primary key) automatically
__str__ decides how a course is shown in the admin and the shell

Common Field Types

Field	Stores
`CharField(max_length=...)`	Short text, like a name
`TextField()`	Long text
`IntegerField()` / `PositiveIntegerField()`	Whole numbers
`DecimalField(max_digits, decimal_places)`	Exact decimals, like money
`BooleanField()`	True or False
`DateField()` / `DateTimeField()`	Dates and times
`EmailField()`	An email address
`SlugField()`	URL-friendly text
`ForeignKey(...)`	A link to another model

Useful options: default=..., blank=True (allowed empty in forms), null=True (allowed empty in the database), unique=True.

Migrations

Migrations turn your model changes into database changes. Run two commands:

Example

bash

python manage.py makemigrations
python manage.py migrate

Output

Migrations for 'courses':
  courses/migrations/0001_initial.py
    + Create model Course
Operations to perform:
  Apply all migrations: admin, auth, contenttypes, courses, sessions
Running migrations:
  Applying courses.0001_initial... OK

Command	What it does
`makemigrations`	Writes a migration file describing your changes
`migrate`	Applies all pending migrations to the database
`showmigrations`	Lists migrations and whether they are applied

Run both commands every time you add, remove or change a model field.

Relationships

A student joins one course; a course has many students. Use ForeignKey:

Example

python

class Student(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField(unique=True)
    course = models.ForeignKey(Course, on_delete=models.CASCADE, related_name="students")
    joined = models.DateField(auto_now_add=True)

    def __str__(self):
        return f"{self.name} ({self.course})"

on_delete=models.CASCADE means: if a course is deleted, delete its students too. Other options are PROTECT (block the delete) and SET_NULL (needs null=True).

Relationship	Field	Example
Many to one	`ForeignKey`	Many students, one course
Many to many	`ManyToManyField`	Students and skills
One to one	`OneToOneField`	A user and a profile

Trying Models in the Shell

Example

bash

python manage.py shell

Example

python

from courses.models import Course, Student

web = Course.objects.create(name="Web Development", fee=8000, duration_months=6)
Student.objects.create(name="Ali", email="ali@example.com", course=web)

print(web.students.count())
print(Student.objects.first())

Output

1
Ali (Web Development)

Exercise

After adding a new field to a model, which two commands update the database?

runserver, then migratemakemigrations, then migratestartapp, then makemigrationsmigrate, then createsuperuser