[FIXED] Django ORM query to update reverse many to many field

Issue

Models look like:

class Book(models.Model):
    name = models.TextField(verbose_name='Book Name')

class Author(models.Model):
    name = models.TextField(verbose_name='Author Name')
    Books = models.ManyToManyField(Book)

Book can have many authors and Author can have written many books

Now,

1. There is a list like:

[
{Book_id1:[Author_id1, Author_id2]},
{Book_id2:[Author_id2, Author_id3]},
]

This information is required to be updated in the DB. How can this be done in an efficient way using ORM?

(Currently the DB may have Book_id1 linked with [Author_id1,Author_id3] so the update should be able to add author_id2 and delete author_id3 as well)

Solution

You can make two queries on the many-to-many model:

data = [
    {book_id1:[author_id1, author_id2]},
    {book_id2:[author_id2, author_id3]},
]
book_ids = [book_id for datum in data for book_id in datum]

BookAuthor = Author.books.through
BookAuthor.objects.filter(book_id__in=book_ids).delete()
BookAuthor.objects.bulk_create([
    BookAuthor(book_id=bi, author_id=ai)
    for datum in data
    for bi, ais in datum.items()
    for ai in ais
])

Answered By - Willem Van Onsem