We’ll do: Book ↔ Category

• One book can have many categories

• One category can contain many books

Tables in TABULAR form with sample records

books

id	author_id	title	published_year	price	is_published
1	1	1984	1949	9.99	true
2	1	Animal Farm	1945	7.99	true
3	2	Harry Potter	1997	19.99	true

---

categories

id	name
1	Fiction
2	Politics
3	Fantasy
4	Classics

---

Join table (auto): myapp_book_categories

(links books ↔ categories)

id	book_id	category_id
1	1	1
2	1	2
3	1	4
4	2	1
5	2	2
6	3	1
7	3	3

This join table is the real many-to-many data.

---

models.py (Many-to-Many)

from django.db import models
from django.core.validators import MinValueValidator

class Author(models.Model):
    name = models.CharField(max_length=200)
    email = models.EmailField(unique=True)
    is_active = models.BooleanField(default=True)
    created_at = models.DateTimeField(auto_now_add=True)

    class Meta:
        db_table = "authors"

    def __str__(self):
        return self.name


class AuthorProfile(models.Model):
    author = models.OneToOneField(
        Author,
        on_delete=models.CASCADE,
        related_name="profile",
    )
    bio = models.TextField(blank=True)
    website = models.URLField(blank=True)
    birth_year = models.PositiveSmallIntegerField(null=True, blank=True)

    class Meta:
        db_table = "author_profiles"

    def __str__(self):
        return f"Profile of {self.author.name}"


# NEW TABLE
class Category(models.Model):
    name = models.CharField(max_length=100, unique=True)

    class Meta:
        db_table = "categories"

    def __str__(self):
        return self.name


class Book(models.Model):
    author = models.ForeignKey(
        Author,
        on_delete=models.CASCADE,
        related_name="books",
    )
    title = models.CharField(max_length=200)
    published_year = models.PositiveSmallIntegerField(validators=[MinValueValidator(1)])
    price = models.DecimalField(max_digits=8, decimal_places=2)
    is_published = models.BooleanField(default=True)

    # MANY-TO-MANY
    categories = models.ManyToManyField(
        Category,
        related_name="books",
        blank=True,
    )

    class Meta:
        db_table = "books"
        constraints = [
            models.UniqueConstraint(
                fields=["author", "title"],
                name="uniq_book_per_author",
            )
        ]

    def __str__(self):
        return self.title

---

Run migrations

python manage.py makemigrations myapp
python manage.py migrate

Practice ORM in Django shell (mini program)

Open shell:

python manage.py shell

from myapp.models import Author, Book, Category

# create author + books
a = Author.objects.create(name="George Orwell", email="orwell@example.com")
b1 = Book.objects.create(author=a, title="1984", published_year=1949, price=9.99)
b2 = Book.objects.create(author=a, title="Animal Farm", published_year=1945, price=7.99)

# create categories
fiction = Category.objects.create(name="Fiction")
politics = Category.objects.create(name="Politics")
classics = Category.objects.create(name="Classics")

# add categories to book (this writes into the join table)
b1.categories.add(fiction, politics, classics)
b2.categories.add(fiction, politics)

# check results
b1.categories.all()
fiction.books.all()

---

Reverse Many-to-Many queries

Book → Categories

b1.categories.all()

Category → Books (reverse)

politics.books.all()

Key takeaways

ManyToManyField = a third table (join table)
Forward: book.categories.all()
Reverse: category.books.all()