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

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.

CodePudding user response：

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
    )

CodePudding user response：

I suggest you to do this:

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

You need one ManyToManyField, then django will create two associative tables between the book and author tables. Changes to one table will not affect the other. You can see in the django documentation about many to many relationships. I have removed your books field in my answer because you have created an instance of your Author but you did not assign his 'book' and you have created a Book instance with giving title and author in your Book model. So so that any of your data won't be deleted. I also added related_name to your author field in your Book model because when you try to get the books related to the author (for example "Loe Tolstoy") you can get by this:

Author.objects.get(name="Leo Tolstoy").books.all()

I hope you understand what I have tried to explain. If any error shows up, let me know by giving the exact error in the comment box. Thanks.