Django ManyToMany model: unable to access one relation from the other

January 16, 2022

My models.py:

class Author(models.Model):
    name = models.CharField(max_length=100)
    books = models.ManyToManyField(
        "Book", related_name="books", blank=True
    )

class Book(models.Model):
    title = models.CharField(max_length=100)
    author = models.ManyToManyField(Author)

In Django admin I first created an instance of Author, without assigning him any Book:

Then I created a new Book and assigned Leo Tolstoy as theAuthor:

In Django shell, I am able to access the Book‘s Author:

>>> Book.objects.filter(title="War and Peace").first().author.first()
<Author: Leo Tolstoy>

But I’m unable to access the Author‘s Books:

>>> Author.objects.filter(name="Leo Tolstoy").first().books.all()
<QuerySet []>

The book isn’t assigned in the Admin view either:

I would like to access the Author‘s Books as well as for the Books
to show in the Author‘s Admin view.

>Solution :

You only need one ManyToManyField. You should also pass "books" as the related_name on Book.authors as this is the name Django will use for the many-to-many manager attribute it automatically adds to Author instances.

See the example in the Django docs for more info.

When a ManyToManyField is created on both sides of a relation, Django is creating two associative tables between the book and author tables under the hood. Changes to one table don’t affect the other.

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

class Book(models.Model):
    title = models.CharField(max_length=100)
    authors = models.ManyToManyField(
        Author, related_name="books", blank=True
    )