Topic: Django Models and ORM — From Basics to Advanced Queries

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What is a Model in Django?

• A model in Django is a Python class that defines the structure of your database table. Each model maps to a table, and each attribute represents a column.

• Django uses its ORM (Object-Relational Mapping) to interact with the database using Python code instead of SQL.

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Creating Your First Model

from django.db import models

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.CharField(max_length=100)
    published_date = models.DateField()
    pages = models.IntegerField()

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Making Migrations

• Create and apply migrations to sync models with the database:

python manage.py makemigrations
python manage.py migrate

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Using the Model

# Creating a new record
book = Book(title="1984", author="George Orwell", published_date="1949-06-08", pages=328)
book.save()

# Fetching all books
books = Book.objects.all()

# Filtering
orwell_books = Book.objects.filter(author="George Orwell")

# Getting one object
book = Book.objects.get(id=1)

# Updating
book.title = "Animal Farm"
book.save()

# Deleting
book.delete()

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Model Field Types

• CharField, TextField, IntegerField, FloatField, DateField, DateTimeField, BooleanField, EmailField, and more.

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Meta Class for Model Options

class Book(models.Model):
    title = models.CharField(max_length=200)

    class Meta:
        ordering = ['title']  # default ordering by title

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Relationships Between Models

• One-to-Many (ForeignKey)
• Many-to-Many (ManyToManyField)
• One-to-One (OneToOneField)

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

class Book(models.Model):
    title = models.CharField(max_length=200)
    author = models.ForeignKey(Author, on_delete=models.CASCADE)

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Advanced ORM Queries

# Complex filters
books = Book.objects.filter(published_date__year__gte=2000, pages__lte=300)

# Exclude
books = Book.objects.exclude(author="J.K. Rowling")

# Ordering
books = Book.objects.order_by("-published_date")

# Count
total = Book.objects.count()

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Summary

• Django models define your database structure.

• The ORM allows you to query and manipulate data using Python.

• Supports relationships, complex filtering, ordering, and aggregation.

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Exercise

• Create two models: Author and Book. Link them using a foreign key. Then, write views that:

1. Add a new book.
2. List all books by a specific author.
3. Delete books published before the year 2000.

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