ORM & Database#

Foreign Key Relationship#

class Category(models.Model):
    name = models.CharField(max_length=100)

class Post(models.Model):
    category = models.ForeignKey(Category, on_delete=models.CASCADE, related_name='posts')

Many-to-Many Relationship#

class Tag(models.Model):
    name = models.CharField(max_length=50)

class Post(models.Model):
    tags = models.ManyToManyField(Tag, related_name='posts')

Basic Queries#

# Create
post = Post.objects.create(title="Hello")

# Read All
posts = Post.objects.all()

# Filter
published = Post.objects.filter(is_published=True)

# Get Single
post = Post.objects.get(id=1)

Advanced Queries (Q Objects)#

from django.db.models import Q

# OR condition
posts = Post.objects.filter(Q(title__icontains='tech') | Q(title__icontains='code'))

Optimization (select_related & prefetch_related)#

# One-to-one or Foreign Key (uses SQL JOIN)
posts = Post.objects.select_related('category').all()

# Many-to-many or Reverse FK (uses separate query)
posts = Post.objects.prefetch_related('tags').all()

Aggregation & Annotation#

from django.db.models import Count, Avg

# Count related items
categories = Category.objects.annotate(num_posts=Count('posts'))

# Aggregate total
avg_rating = Post.objects.aggregate(Avg('rating'))

Migrations Flow#

# Create migration files
python manage.py makemigrations

# Apply migrations
python manage.py migrate

# Check status
python manage.py showmigrations