How to update manytomany field in Django?

Question

Here'a an example:

If I have these classses

class Author(models.Model):
    name = models.CharField(max_length=45)

class Book(models.Model):
    name = models.CharField(max_length=45)
    authors = models.ManyToManyField(Author)

In the database I have one Author with the name "George" and another one with the name "Georfe". The last one is a mistake. So what I want is in every Book that have "Georfe" as one of his Author replace it by the Author "George".

In SQL is really easy to do. If id of "George" = 3 and id of "Georfe" = 7 and the relation table name is "author_book":

UPDATE author_book SET id=3 WHERE id=7;

Is it possible to do that with the Django ORM?

I found a way to do it: I loop trough all the related Books of the mistyped author and do:

book.authors.add(Autor.objects.get(id=3))
book.authors.remove(Author.objects.get(id=7))

But I don't find this solution really elegant and efficient. Is there a solution without the loop?

Answer 1

With the auto-generated through table, you have to do a two-step insert and delete.

george = Author.objects.get(name='George')
georfe = Author.objects.get(name='Georfe')

book.authors.add(george)
book.authors.remove(georfe)
assert george in book.authors

If you have an explicitly defined through table (authors = models.ManyToManyField(Author, through=BookAuthors) then you can change the relationship explicitly on BookAuthor.

book_author = BookAuthor.objects.get(author=georfe, book=great_american_novel)
book_author.author = george
book_author.save()
assert george in book.authors

If you don't have an explictly defined through table, you don't lose anything by adding and deleting explicitly, except a bit of efficiency, but this is a one-time clean up task, so simplicity should trump efficiency. If you're concerned about it, as the commenter said, drop to SQL. That's what it's there for.

Answer 2

Can you do something like this?

george_author = Author.objects.get(name="George")
for book in Book.objects.filter(authors__name="Georfe"):
    book.authors.add(george_author.id)
    book.authors.filter(name="Georfe").delete()

I suspect that this would be easier if you had an explicit table joining the two models (with the "through" keyword arg) -- in that case, you would have access to the relationship table directly, and could just do a .update(id=george_author.id) on it.