I'm working on a Django project and encountering an issue with Django authentication. Here's my current implementation:

# models.py
class Article(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(User, on_delete=models.CASCADE)
    
    def save(self, *args, **kwargs):
        # This is causing issues
        super().save(*args, **kwargs)

The specific error I'm getting is: "django.db.utils.DataError: value too long for type character varying(100)"

I've already tried the following approaches:

• Checked Django documentation and Stack Overflow
• Verified my database schema and migrations
• Added debugging prints to trace the issue
• Tested with different data inputs

Environment details:

• Django version: 5.0.1
• Python version: 3.11.0
• Database: PostgreSQL 15
• Operating system: Windows 11

Has anyone encountered this before? Any guidance would be greatly appreciated!

This Django error typically occurs when you're trying to save a model instance that violates a unique constraint. Here's how to handle it properly:

from django.db import IntegrityError
from django.http import JsonResponse

try:
    user = User.objects.create(
        username=username,
        email=email
    )
except IntegrityError as e:
    if 'username' in str(e):
        return JsonResponse({'error': 'Username already exists'}, status=400)
    elif 'email' in str(e):
        return JsonResponse({'error': 'Email already exists'}, status=400)
    else:
        return JsonResponse({'error': 'Data integrity error'}, status=400)

Always use get_or_create() when you want to avoid duplicates:

user, created = User.objects.get_or_create(
    username=username,
    defaults={'email': email, 'first_name': first_name}
)

To handle Django database transactions properly and avoid data inconsistency, use Django's transaction management:

from django.db import transaction

# Method 1: Decorator
@transaction.atomic
def transfer_money(from_account, to_account, amount):
    from_account.balance -= amount
    from_account.save()
    
    to_account.balance += amount
    to_account.save()

# Method 2: Context manager
def complex_operation():
    with transaction.atomic():
        # All operations in this block are atomic
        user = User.objects.create(username='test')
        profile = UserProfile.objects.create(user=user)
        # If any operation fails, all are rolled back

For more complex scenarios with savepoints:

def nested_transactions():
    with transaction.atomic():
        # Outer transaction
        user = User.objects.create(username='test')
        
        try:
            with transaction.atomic():
                # Inner transaction (savepoint)
                risky_operation()
        except Exception:
            # Inner transaction rolled back, outer continues
            handle_error()

To optimize Django QuerySets and avoid N+1 problems, use select_related() for ForeignKey and OneToOneField, and prefetch_related() for ManyToManyField and reverse ForeignKey:

# Bad: N+1 query problem
for book in Book.objects.all():
    print(book.author.name)  # Each iteration hits the database

# Good: Use select_related for ForeignKey
for book in Book.objects.select_related('author'):
    print(book.author.name)  # Single query with JOIN

# Good: Use prefetch_related for ManyToMany
for book in Book.objects.prefetch_related('categories'):
    for category in book.categories.all():
        print(category.name)  # Optimized with separate query

You can also use only() to limit fields and defer() to exclude heavy fields:

# Only fetch specific fields
Book.objects.only('title', 'author__name').select_related('author')

# Defer heavy fields
Book.objects.defer('content', 'description')