Django QuerySet Introduction

Key QuerySet Features

books = Book.objects.all()  # No query yet
print(books)  # Query executes now

books = Book.objects.filter(
    published=True
).order_by('-date')

# Django creates efficient SQL
books = Book.objects.filter(
    price__lt=50
)

for book in Book.objects.all():
    print(book.title)

🔹 Creating Your First QuerySet

QuerySets are created using the model's Manager, which is accessed through the objects attribute. Every Django model automatically gets a Manager that provides QuerySet methods for database operations.

# models.py
from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.CharField(max_length=100)
    price = models.DecimalField(max_digits=6, decimal_places=2)
    published = models.BooleanField(default=False)

# views.py
from myapp.models import Book

# Create a QuerySet
all_books = Book.objects.all()
print(all_books)

🔹 QuerySet Methods Overview

Django provides many methods to work with QuerySets. These methods allow you to retrieve, filter, and manipulate data from your database efficiently without writing raw SQL queries.

# Common QuerySet methods
from myapp.models import Book

# Get all objects
all_books = Book.objects.all()

# Get single object
book = Book.objects.get(id=1)

# Filter objects
published_books = Book.objects.filter(published=True)

# Exclude objects
unpublished = Book.objects.exclude(published=True)

# Order objects
ordered_books = Book.objects.order_by('title')

# Count objects
book_count = Book.objects.count()

🔹 QuerySet Evaluation

QuerySets are lazy - they don't query the database until you actually need the data. This happens when you iterate, slice, print, or call certain methods on the QuerySet.

# QuerySet is created but not evaluated
books = Book.objects.filter(published=True)

# Now it's evaluated (hits database)
for book in books:
    print(book.title)

# These also trigger evaluation:
list(books)  # Convert to list
len(books)   # Get count
books[0]     # Access by index
bool(books)  # Check if exists

🔹 Basic QuerySet Example

Here's a complete example showing how to use QuerySets in a Django view to retrieve and display data from your database.

# views.py
from django.shortcuts import render
from myapp.models import Book

def book_list(request):
    # Get all published books
    books = Book.objects.filter(published=True)
    
    # Pass to template
    context = {'books': books}
    return render(request, 'books.html', context)

# In template (books.html)
# {% for book in books %}
#     <h3>{{ book.title }}</h3>
#     <p>by {{ book.author }}</p>
# {% endfor %}