django models and migrations

Tutorial 10 - Django Models and Migrations

Why this matters

Applications need to remember information.

A booking application remembers bookings. An inventory application remembers products. A CRM remembers customers.

Django models describe this data in Python.

1. A model

from django.db import models

class Product(models.Model):
    name = models.CharField(max_length=100)
    price = models.DecimalField(
        max_digits=10,
        decimal_places=2,
    )

This describes a product record.

Conceptually:

Product
 ├── name
 └── price

2. Common fields

You will commonly encounter:

models.CharField()
models.TextField()
models.IntegerField()
models.DecimalField()
models.BooleanField()
models.DateField()
models.DateTimeField()
models.EmailField()

For example:

class Booking(models.Model):
    name = models.CharField(max_length=100)
    email = models.EmailField()
    date = models.DateField()
    confirmed = models.BooleanField(default=False)

3. The database connection

A relational database stores records in tables.

Your model:

class Product(models.Model):
    name = ...
    price = ...

corresponds conceptually to:

product table

id | name     | price
---|----------|------
1  | Laptop   | 50000
2  | Mouse    | 1000

Django's ORM lets you work with these records using Python.

4. What is a migration?

Changing models.py does not automatically change the database.

Suppose you add:

description = models.TextField()

Your Python model now expects a new field.

The database also needs a structural change.

Migrations are Django's mechanism for recording and applying those changes.

5. Creating migrations

Run:

python manage.py makemigrations

Django examines your models and creates a migration file.

You might see:

Migrations for 'main':
  main/migrations/0002_add_description.py

6. Applying migrations

Then:

python manage.py migrate

Think:

makemigrations
 ↓
Create instructions

migrate
 ↓
Apply instructions

7. Why you might see "no such table"

Suppose your code executes:

Product.objects.all()

and Django reports:

django.db.utils.OperationalError:
no such table: main_product

The likely situation is:

Python model exists
        ↓
Database table does not

Check:

python manage.py showmigrations

Then:

python manage.py migrate

If you changed a model:

python manage.py makemigrations
python manage.py migrate

8. Creating records

Product.objects.create(
    name="Mouse",
    price=1000,
)

9. Reading records

All:

Product.objects.all()

One:

Product.objects.get(id=1)

Filtered:

Product.objects.filter(price__gt=1000)

This is Django's ORM.

10. Updating

product = Product.objects.get(id=1)
product.price = 1200
product.save()

The save() writes the change to the database.

11. Deleting

product.delete()

Deletion should be treated carefully because data can be lost.

12. Migrations and Git

Migration files normally belong in Git:

main/migrations/
    0001_initial.py
    0002_add_description.py

They describe database changes.

The local database itself should generally not be treated as source code.

13. Exercise

Create:

class Service(models.Model):
    name = models.CharField(max_length=100)
    price = models.DecimalField(
        max_digits=10,
        decimal_places=2,
    )

Then:

python manage.py makemigrations
python manage.py migrate

Register it in Admin.

Create a record.

Display it through a view and template.

Trace:

Model
 ↓
Migration
 ↓
Database
 ↓
Admin
 ↓
View
 ↓
Template

14. Using ChatGPT

If migrations fail, provide the exact error, models.py, migration output, and showmigrations output.

Ask:

Explain why my Django model and database schema are out of sync and give me the smallest safe fix.

Do not delete migrations or databases just because an AI suggests it without explaining why.

Remember

models.py
 ↓
makemigrations
 ↓
migration files
 ↓
migrate
 ↓
database schema

Models describe data. Migrations keep database structure synchronized with those models.