django

Django

Django is a high-level Python web framework encouraging rapid development, pragmatic clean design, built-in ORM security, robust migrations, and asynchronous view support.

When to Use

• Full-Featured Python Web Applications: Batteries-included web development with built-in ORM, admin panel, auth, and migrations.
• Content Management & Enterprise Backends: Developing robust database-backed backends with automatic admin dashboards.
• High-Throughput REST APIs: Building secure APIs using Django REST Framework (DRF) or Django Ninja.
• Async Python Web Services: Utilizing ASGI, async views, and background tasks on Django 5.x.

Quick Start

# models.py
from django.db import models

class Post(models.Model): title = models.CharField(max_length=200) # New in 5.0: Database generated field slug = models.GeneratedField( expression=models.functions.Concat(models.F("title"), models.Value("-slug")), output_field=models.CharField(max_length=205), db_persist=True, )

## Core Concepts

### Django ORM Models & Migrations

Relational schema definition with indexing and query optimizations:

```python
from django.db import models
from django.utils import timezone

class Customer(models.Model):
    email = models.EmailField(unique=True, db_index=True)
    full_name = models.CharField(max_length=255)
    created_at = models.DateTimeField(default=timezone.now)

    class Meta:
        ordering = ['-created_at']

    def __str__(self):
        return f"{self.full_name} ({self.email})"

class Order(models.Model):
    customer = models.ForeignKey(Customer, on_delete=models.CASCADE, related_name='orders')
    total_amount = models.DecimalField(max_digits=10, decimal_places=2)
    is_fulfilled = models.BooleanField(default=False)
    placed_at = models.DateTimeField(auto_now_add=True)

    class Meta:
        indexes = [
            models.Index(fields=['customer', 'is_fulfilled']),
        ]

High-Performance Querying with select_related & prefetch_related

Eliminating N+1 database queries:

from django.shortcuts import render
from .models import Order

def order_report_view(request):
    # select_related performs SQL JOIN for 1-to-1 and ForeignKey
    # prefetch_related performs separate batch query for ManyToMany
    orders = Order.objects.filter(is_fulfilled=True) \
                          .select_related('customer') \
                          .only('id', 'total_amount', 'customer__full_name')[:50]

    return render(request, 'orders/report.html', {'orders': orders})

Fast Async APIs with Django Ninja

Type-safe REST API endpoints powered by Pydantic:

from ninja import NinjaAPI, Schema
from typing import List

api = NinjaAPI(title="Inventory API", version="1.0.0")

class ProductOut(Schema):
    id: int
    name: str
    price: float
    in_stock: bool

@api.get("/products", response=List[ProductOut])
async def list_products(request):
    # Async view querying ASGI backend
    return [
        {"id": 1, "name": "Mechanical Keyboard", "price": 129.99, "in_stock": True}
    ]

Common Patterns

Prefetching Related Objects to Prevent N+1 Queries

Problem: Accessing foreign keys or many-to-many relationships in template/API loops generates hundreds of database queries.

Solution: Use select_related for ForeignKeys and prefetch_related for M2M:

# Fetches books and authors in a single optimized JOIN query
books = Book.objects.filter(is_published=True)    .select_related("publisher")    .prefetch_related("authors")    .order_by("-published_date")[:50]

for book in books:
    print(f"{book.title} published by {book.publisher.name}")

Best Practices

Do:

• Always use select_related and prefetch_related to eliminate N+1 database query bottlenecks.
• Configure django-environ to keep secret keys, database credentials, and API tokens out of code.
• Run migrations in CI pipelines (python manage.py check --deploy and makemigrations --check).
• Use Bulk operations (bulk_create, bulk_update) when modifying more than 10 records.

Don't:

• Leave DEBUG = True in production environments; it leaks memory and sensitive stack traces.
• Put business logic inside templates or views; encapsulate domain rules in model methods or service layers.
• Use synchronous external HTTP calls in view handlers; use async views or Celery/Redis tasks.

Troubleshooting

Error	Cause	Solution
django.db.utils.OperationalError: no such table	Database migrations have not been applied.	Run python manage.py migrate.
CSRF verification failed. Request aborted (403)	POST form submitted without {% csrf_token %} tag.	Include {% csrf_token %} in HTML form or verify CSRF cookie headers in API requests.
FieldError: Cannot resolve keyword '...' into field	Misspelled field name in filter() or order_by().	Check model definition and ensure double underscores __ for relationship lookups.

References

• Django Documentation