Topic: Django ORM – Transactions, Subqueries, and Custom Managers (Part 3)

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1. Working with Transactions

• Django supports atomic transactions to ensure database integrity — either all operations succeed, or none do.

from django.db import transaction

@transaction.atomic
def create_author_and_book():
    author = Author.objects.create(name="New Author")
    Book.objects.create(title="New Book", author=author)

• Use atomic() as a decorator or context manager.

with transaction.atomic():
    # multiple operations that must succeed together
    ...

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2. Subqueries and OuterRef

• Use Subquery and OuterRef to perform queries that depend on other queries.

from django.db.models import Subquery, OuterRef

# Get latest book for each author
latest_books = Book.objects.filter(author=OuterRef('pk')).order_by('-published_date')
authors = Author.objects.annotate(latest_book=Subquery(latest_books.values('title')[:1]))

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3. Exists() and Conditional Logic

• Use Exists for optimized existence checks.

from django.db.models import Exists

recent_books = Book.objects.filter(published_date__year=2023)
authors = Author.objects.annotate(has_recent_books=Exists(recent_books.filter(author=OuterRef('pk'))))

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4. Custom Model Managers

• Add custom query logic to models via custom managers.

from django.db import models

class PublishedBookManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().filter(is_published=True)

class Book(models.Model):
    title = models.CharField(max_length=200)
    is_published = models.BooleanField(default=False)

    objects = models.Manager()  # Default manager
    published = PublishedBookManager()  # Custom manager

# Usage
Book.published.all()

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5. QuerySet Methods: Update, Delete, Bulk Operations

• update() modifies multiple records efficiently.

Book.objects.filter(author__name="Alice").update(pages=300)

• delete() removes objects in bulk.

Book.objects.filter(published_date__year__lt=2000).delete()

• bulk\_create() inserts many records at once.

Book.objects.bulk_create([
    Book(title="Book A", author=author),
    Book(title="Book B", author=author),
])

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6. Using Database Functions

• Django provides built-in SQL functions like Lower, Upper, Length, Concat, etc.

from django.db.models.functions import Upper

books = Book.objects.annotate(upper_title=Upper('title'))

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Summary

• Use transactions to maintain data integrity.

• Leverage subqueries, OuterRef, and Exists for complex logic.

• Create custom managers to encapsulate reusable query logic.

• Apply bulk operations and DB functions for performance and flexibility.

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Exercise

• Create a custom manager for the Book model to return only books published in the last 5 years. Then use this manager in a view to list all recent books along with their authors.

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#Django #ORM #Transactions #Subqueries #CustomManagers #AdvancedDjango

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