Django Architecture Cheat Sheet: ORM, Signals, Sessions, and Auth

# Query basic datasets
Article.objects.all()
Article.objects.all().count()
Article.objects.filter(is_published=True).exists()

# Slicing (LIMIT and OFFSET)
Article.objects.filter(is_published=True)[:5]

# Specific field projections
Article.objects.filter(is_published=True).values('title', 'created_at')[:5]

# Ordering
Article.objects.filter(is_published=True).order_by('-created_at')

# Field lookups and date filtering
from datetime import datetime
Article.objects.filter(created_at__gte=datetime(2025, 1, 1))
Article.objects.filter(created_at__year=2025).order_by('id')

# Inspect underlying compiled SQL
query = Article.objects.filter(is_published=True)
print(str(query.query))

for article in Article.objects.raw("SELECT id, title FROM myapp_article WHERE is_published = %s", [True]):
    print(article.id, article.title)

from django.db import connection

def fetch_custom_report():
    with connection.cursor() as cursor:
        cursor.execute("SELECT category, COUNT(*) FROM myapp_article GROUP BY category")
        rows = cursor.fetchall()
    return rows

from django.db.models.signals import post_save
from django.dispatch import receiver
from myapp.models import Article

@receiver(post_save, sender=Article)
def notify_subscribers_on_publish(sender, instance, created, **kwargs):
    if created and instance.is_published:
        print(f"[+] New article published: {instance.title}")

from django.apps import AppConfig

class MyAppConfig(AppConfig):
    default_auto_field = 'django.db.models.BigAutoField'
    name = 'myapp'

    def ready(self):
        import myapp.signals

from django.dispatch import Signal

# Define a custom signal
payment_verified = Signal()

# Dispatch the signal in a view or service
payment_verified.send(sender=Order, order_id=1042, amount=99.00)

# Connect a listener handler
@receiver(payment_verified)
def handle_payment_completed(sender, **kwargs):
    print(f"Receipt issued for order {kwargs.get('order_id')}")

def user_dashboard(request):
    # Set session variable
    request.session['has_visited_dashboard'] = True
    
    # Retrieve session variable with fallback
    theme_preference = request.session.get('theme', 'dark')
    
    # Check session existence
    if 'auth_token' in request.session:
        # Perform authenticated tasks
        pass

from django.contrib.auth.backends import ModelBackend
from django.contrib.auth import get_user_model

UserModel = get_user_model()

class EmailAuthBackend(ModelBackend):
    """
    Authenticate users using an email address instead of a username.
    """
    def authenticate(self, request, username=None, password=None, **kwargs):
        email = kwargs.get('email', username)
        if email is None:
            return None
        try:
            user = UserModel.objects.get(email=email)
        except UserModel.DoesNotExist:
            # Run default password hasher to mitigate timing attacks
            UserModel().set_password(password)
            return None
        
        if user.check_password(password) and self.user_can_authenticate(user):
            return user
        return None

AUTHENTICATION_BACKENDS = [
    'myapp.backends.EmailAuthBackend',
    'django.contrib.auth.backends.ModelBackend',
]